Hard Drive 101

How does a hard drive work?

drive platter

Figure 1 The inside of a hard disc drive, showing the disk platter and the read/write head.

While the disk platter looks like a mirror, it’s actually composed of up to trillions of tiny magnets standing on end, arrayed in concentric circles. The polarity of each magnet can be “up” or “down,” which indicates whether the bit is a 1 or a 0. The read/write head moves like a record tone arm, and can flip the polarity of the magnet when it’s writing data, or read the polarity when it’s reading data.

The magnets in a hard disk are organized in concentric circles — as many as 250,000 rings on a 3.5-inch platter. The head skims back and forth at up to 10 meters/second and must stop on a line 1/10 the width of a human hair, and then correctly read the polarity of each bit. It’s amazing that this is even possible, and even more amazing that it’s affordable.

A hard disk drive also has electronics to control the mechanism, to translate the data to a format that can be written to the disk and to do error correction and analysis. Hard drives have a power connector that provides juice for the motor that spins the drive and for the controller circuitry. Each drive also has a data interface: IDE/ATA or SATA for desktop drives, and Serial Attached SCSI (SAS) or Fibre Channel for enterprise drives.

Hard drive sizes

Hard drives come in two basic physical sizes: 2.5-inch and 3.5-inch. These sizes refer to the size of the data platters, not the size of the hard drive mechanism. Traditionally, 2.5-inch drives are used for laptops while 3.5-inch drives are used for desktop computers. Some compact desktops also use the smaller drives to enable a smaller form factor for the computer.

3.5 and 2.5 inch drives

FIGURE 2 shows the two sizes of drives generally in use. 3.5 inch drives, on the right, are used in desktop computers and in freestanding storage devices. 2.5 inch drives are used in laptops and portable storage devices. Newer 2.5 inch drives are also being used in high-performance storage devices.

2.5-inch drives generally spin slower which means that they have slower data throughput. They also have a smaller data capacity and are more expensive per gigabyte. The smaller drives do have several advantages depending on the use.

  • They are physically smaller so they can fit in laptops and small portable enclosures.
  • They may have better “seek” times, since the read head has less distance to travel than with a larger diameter drive.

    They need less power to spin so they can generally be bus-powered, meaning they can draw power from a laptop without the use of an external power supply.

    And since they are designed to be portable, most of them do a better job of “parking the heads” than full-size drives do. This means they are better able to survive being shipped around or used in a moving environment.

    Recent developments in 2.5-inch drives are changing how the small drives are used. A new class of 2.5-inch high-speed drives has emerged that can be used in enterprise and server environments. At the moment these drives are very expensive per gigabyte.

Solid-state drives (SSD)


FIGURE 3 Solid-state drives hold a number of advantages over spinning disks.

A new kind of storage device for computers has shown up in the marketplace over the last several years. Instead of spinning disks, solid-state flash memory is being used as primary storage. It offers a number of advantages, particularly for use in portable computing, and for speeding up certain kinds of data storage and access.

Read more about SSDs in this section

Hard drive capacity

The capacity of a hard drive refers to the amount of data it can hold. These days, capacity is measured in gigabytes or terabytes. Due to marketing reasons, the capacities listed on drive specifications may not be calculated in the same way that your operating system calculates data sizes.

For instance, a drive sold as “500GB” actually only contains 465GB (actually, the 500 number is gibibytes, and the 465 number is gigabytes. Aren’t you glad you asked?) Windows continues this practice, but Mac OS 10.6 and later changed the way it calculates size to match manufacturers’ practice.

For most still photographers, we generally suggest that it’s better to get the largest capacity drives you’re likely to need, at least for the next 6-12 months (if you’re on a RAID system, you’ll want a longer time frame — maybe two years — due to the complexity of upgrade). Running fewer drives saves on space, power draw and heat generation. It’s also easier to manage your drives if there are fewer of them.

For high-volume photographers and videographers, the issue can be significantly more complex. Storage needs for individual projects will easily climb to hundreds of gigabytes or into terabytes. If you are in this situation, acquiring hard drive capacity may resemble the model that was used back in the days of tape or film stock. Instead of keeping a general archive, you may have to factor the cost of storage into the price of each project, and buy the drives/tape/discs on a per-job basis.

It’s also possible that you need to go to a Storage Area Network (SAN) model for storage, where enterprise-class servers manage a large tiered storage pool.


Should I use big drives or small drives?

One question comes up over and over. Is it better to have your primary storage on (fewer) bigger drives or (more) smaller ones? If you chose big drives, a single drive failure can take out a lot of files, so it might seem like you get more protection with a larger number of smaller drives. We don’t agree.

All your digital storage should be configured so that failure of any one drive does not kill the only copy of any files. You must backup the images to an additional device if you want to preserve them.

If you use a smaller number of larger drives for storage, you will simplify the process of keeping track of the drives, as well as the process for periodically checking on the integrity of your data. You’ll also use less energy to keep them spinning and save on storage or desktop space. Additionally, larger drives are likely to be newer and faster.

Hard drive rotation speeds

As part of its specifications, each hard drive has a speed at which the platter rotates, measured in RPMs. The faster the drive, the faster the throughput, since the head reads and writes the bits at a faster rate.

2.5-inch consumer drives typically spin at 4200, 5200, 5400 and 7200 RPMs. 7200 RPM drives are a good choice at the moment, but sometimes 7200 RPM drives have too large a power draw or generate too much heat for the portable devices in which they are housed. The enterprise-class 2.5-inch drives currently spin at 10,000 or 15,000 RPMs.

3.5-inch drives generally come in 5200, 7200, 10,000 and 15,000 RPM models. The 7200 RPM models are good all-purpose drives and have the largest capacities. The faster drives are generally used for system or scratch disks, where fast disk-swapping speeds up the performance of programs like Photoshop, which often have to work with large files.

You can also purchase variable-speed 3.5 inch drives, typically sold as “energy-saving” or “green” drives, running between 5400 and 7200 RPMs. These drives use less energy and have slower data transfer rates. This makes them a reasonable choice for Archives and for off-line backups.

Hard drive interfaces

Hard drives come with one of several different connectors built in. When you buy a drive, it will specify which one is built into the drive. The five types are ATA/IDE and SATA for consumer-level drives, and SCSI, Serial Attached SCSI (SAS), and Fibre Channel for enterprise-class drives.


For many years, Advanced Technology Attachment (ATA) connections were the favored internal drive connection in PCs. Apple adopted ATA with the Blue and White G3 models. ATA drives must be configured as either a master or a slave when connecting. This is usually accomplished by the use of a hardware jumper or, more recently, through the use of a cable that can tell the drive to act as either a master or slave.

ATA also goes by the name ATAPI, IDE, EIDE and PATA, which stands for Parallel ATA. ATA is still in use in many computers today, but most drive manufacturers are switching over to SATA (Serial ATA). If you have any devices that still use PATA drives, that’s a good clue that you’re in need of an upgrade.


As of 2007, most new computers (Macs and PCs, laptops and desktops) use the newer SATA interface. It has a number of advantages, including longer cables, faster throughput, multidrive support through port multiplier technology, and easier configuration. SATA drives can also be used with eSATA hardware (discussed later) to enable fast, inexpensive configuration as an external drive. Most people investing in new hard drive enclosures for photo storage should be using SATA drives.

SCSi/SAS and Fibre Channel

SCSI, SAS, and Fibre Channel drives are rare in desktop computers, and are typically found in expensive enterprise-level storage systems. You can also find SAS drives (along with the necessary SAS controller cards) in video editing systems where maximum throughput is needed.

Some of the faster drives, such as Western Digital Raptors, come with SAS connectors, so be aware when you mail-order one. Standard SATA drives can be connected to an SAS controller, but SAS drives can't be connected to a standard SATA controller.

Hard drive enclosures

Now that we’ve gone over some characteristics of hard drive mechanisms, let’s consider where the drive can live. The enclosure for your hard drive can be the computer itself (for an internal drive), a single-drive external case, or a multiple- drive external case.

Internal drives

If you are using a tower computer to store your archive, it is likely that you have one or more empty drive bays inside the computer that can hold a new drive. Some advantages of using internal drives are that they are the cheapest way to add storage and they take up the least amount of room. They are also capable of connecting directly to the computer’s logic board, so they provide fast access. One drawback is that they aren’t as easy to swap out as external drives.

Single-drive externals

Single Drive Enclosure

Figure 4 Adding a single-drive external enclosure is an easy way to add storage to your computer system.

If you don’t have an empty drive bay, or if installing a new internal drive seems too daunting, it is usually very easy to add external drives to your computer using FireWire (IEEE1394 or IEEE1394b), USB (2 or 3), Thunderbolt, or eSATA connections. External single-drive cases have the advantages of being easily portable and not increasing the demand on your computer’s cooling system. The drawbacks are the higher cost and extra clutter.

You can get single-drive externals in two ways.

  • You can purchase an external drive as a ready-made unit. These devices offer a quick and economical way to add storage to your system, but they often come with a shorter warranty than a bare drive, and oftentimes these drives suffer from poor throughput. Manufacturers will often sell their lowest performing drives in external cases.
  • You can also purchase a freestanding enclosure and an internal drive and put them together, like the one pictured in Figure 4. We like this option because it offers more control over the components and because we can reuse the case when we outgrow the capacity of the drive.

Multiple-drive externals

Multiple-drive cases are an excellent solution for a large archive. Although they are larger, there’s less wiring clutter than with several single-drive cases. And once you have bought a big drive box, you can fill it with less-expensive internal drives, which you can later swap out for higher capacity drives as additional space is required. This is the arrangement that we currently favor.

4 drive externals

FIGURE 5 shows a four-bay external drive enclosure. This is a trayless model for SATA drives. These units provide an easy way to add more storage to your computer.

External hard drive interfaces

The hard drive mechanism has its internal interface (PATA, SATA, SAS, or Fibre Channel), and the enclosure has one or more external interfaces as well. The external interface determines how the drive enclosure connects to the computer. There are four principal ones in use, and a few additional ones that are used in high-end systems. Figure 6 shows a drive that has the three most common connection types.

external Hard Drive Interfaces

FIGURE 6This photograph shows an external drive with all the most common interfaces.


USB is the most universal connection method for adding peripheral devices to computers. On the PC, USB 2 (stay away from USB 1 because of its slow speeds) is a good way to connect external drives. Data throughput maxes out at a theoretical 30 megabytes per second per device, in most cases. Due to the USB drivers in the Mac OS, USB is considerably slower on Apple machines. USB 3.0 version was recently released and offers a tenfold increase in theoretical performance. USB connectors can supply bus power to attached devices.

Multiple USB devices can be connected to a single port by means of an external hub.


FireWire 400 and FireWire 800 (also known as IEEE1394 and IEE1394b) are more modern connection protocols than USB, with theoretical transfer maximums of 50 and 100 megabytes per second. FireWire devices can be daisy chained, allowing the use of multiple drives on a single port. Like USB, implementations differ between Mac and PC, with Mac generally making greater use of the speed capabilities than PC. FireWire can also offer bus power to run external drives if the FireWire port is a four-pin, six-pin or nine-pin port. (Many PCs only offer four-pin ports.)

Multiple devices may be connected to a single FireWire port, by means of “daisy-chain” connection from one FireWire device to another.


eSATA is a configuration that creates a SATA connection in an external enclosure. It’s generally a fast and stable connection, offering up to 150, 300 or 600 megabytes per second. eSATA is relatively common as a built-in external connection on PC, but is not built in to any Apple computers. You can add eSATA to Apple computers and older PCs by means of an expansion card, such as Peripheral Component Interface express(PCIe) for desktops and ExpressCard for some laptops.

Conventional eSATA does not have the capability to bus-power hard drives so you must use an external power source. We are starting to see some Powered eSATA drives on the market, but they are rare.

eSATA is often described as hot-swappable, meaning that you can disconnect and reconnect different drives without restarting the computer, but this is often not the case. The design of the host (the way the eSATA is connected to the logic board) will determine if the connection really is hot-swappable.

Multiple eSATA devices can be connected to a single port if the port supports “Port Multiplication”.


In 2011, Apple released the first computers with a built-in Thunderbolt connection. This interface supports multiple streams of high-resolution video as well as multiple streams of fast data using the Mini DisplayPort connector. The Thunderbolt standard supports external storage devices as well as external monitors. The data connectivity of Thunderbolt is based on the same kind of PCIe connection that is used with expansion cards on tower computers – basically, it offers a direct connection to the logic board for unsurpassed speed.

The standard also supports the use of adapter cables that allow FireWire, USB and eSATA devices to be plugged into Thunderbolt ports. At the time of this writing, Thunderbolt accessories, cables and peripherals are rare, probably due to the low supply of Thunderbolt chipsets that are needed to provide the Thunderbolt connection.

Up to seven devices (Including monitors in that count) can be connected by daisy-chain to a Thunderbolt port.

Figure 7The Thunderbolt connection carries both video and data over a single tiny connector.


Internet Small Computer System Interface (iSCSI) is a connection method that uses existing Ethernet hardware to attach the storage to the computer. An iSCSI device can be attached directly to a computer's network port, or a router or switch can connect it. It's fast and flexible, and offers throughput in the neighborhood of 120 MB/s.

Note that iSCSI needs "initiator" software that manages the connection. Some devices, such as the DroboPro shown in Figure 8 include this software. Other iSCSI device manufacturers suggest you purchase separate iSCSI initiator software.

Drobo iSCSI

FIGURE 8shows the connectors on a DroboPro unit. From left to right they are USB, Firewire 800 and iSCSI.


SAS connections can be internal or external. This fast connection is found mostly on enterprise-level hardware, like dedicated servers, RAID, and tape drive mechanisms. Throughput for SAS devices is similar to SATA 2 or 3, in the neighborhood of 300 or 600 MB/s.

Fibre Channel

Fibre Channel is a technology that has migrated from supercomputers down to enterprise-level storage (big companies). It offers a high throughput and the potential to be used over distances of several hundred feet. It can be used over copper cable as well as optical fiber. It is rated at up to 3.2 GB/s.

Choosing the right hard drive and connection

When you add external storage to your computer, you’ll want to make sure it’s fast enough for the task at hand. Sometimes, speed won’t be terribly important, such as backup storage for your Archive files. Sometimes speed will be critically important, such as primary storage for video source files. In most cases, it’s not hard to know when your storage speed is the workflow bottleneck. Downloads and transfers will take too long, or Photoshop will seem to stop as you hear the hard drives grinding away.

Choosing the right speed of drive and a drive connection is not terribly hard, but the specifications that are published can be misleading. Sometimes, manufacturers will use the speed of a connection port as the listed speed of the device, when the actual drive is much slower than that. And many connection types don’t actually live up to the listed speed. USB 2, for instance, specifies a transfer rate of 60 MB/s. But that’s really for two devices on the same USB port, and there are almost no single devices that will perform faster than 30 MB/s.

Match the connection speed to the drive speed

There’s no point in paying a lot extra for a fast connection if the drive delivers data at a small fraction of the speed. And there’s no point in setting up fast disks and connecting them with a too-slow connection. The chart in Figure 9 outlines some rough data transfer rates for drive types and for connection types.

Mbps and MB/s

When you look at drive speed figures, you will often see two different notations that look very similar. Megabits per second is written as Mbps, and megabytes per second is typically written as MB/s. There are 8 bits per byte, so the relationship between the two is exactly 8:1. It’s the same with gigabits (Gb) and gigabytes (GB). When the b is lower case, the notation is bits, when it is capitalized, it is bytes. Since most of us think in bytes rather than bits, that’s the one we’ll use for comparison.

For instance, FireWire 400 is named for the number of megabits that can be transferred in a second, which is about 400. Divide that by 8 to get the number of megabytes that can be transferred in a second: 50. (It’s actually just a little bit less: 393 Mbps and 49 MB/s).

Of course, a gigabyte is 1000 megabytes, so once measurements get above 1000 MB/s, we change to GB/s.

Note that there is a difference between the rated speed and the typical real-world speed. All connections provide slower actual throughput than the rated speeds – some significantly so. Check the chart in Figure 9 to get a better idea of actual speed.

Hard drives almost never achieve maximum throughput

It’s very difficult to outline real-world speed for a drive. Hard drives are slowed down considerably when they read or write small files. Data on the outer rings of a drive platter is read faster than data on the inner rings. And as a drive fills up, things slow down even more.

A single 7200 RPM drive, for instance, should outperform a FireWire 800 connection, since peak data transfer is typically above FireWire 800’s 98 MB/s. But you will only find that happening in rare circumstances – in most cases, the drive will be serving up data at a significantly slower rate.

Bigger files transfer much faster than smaller files

When you transfer a big file, your drive can spend most of its time actually reading or writing data, so it works at its most efficient pace. When you transfer smaller files, the drive spends a lot more time “seeking” the files – moving the head to the part of the data platter that contains the files.

SSDs are able to do a much better job with small files, since no parts need to move to the place the data is stored, but smaller files still slow SSDs down. That’s because there’s a certain amount of administrative overhead associated with each file read or write.

Bigger drives are usually faster

There are several reasons that larger capacity drives are usually faster than a comparable RPM drive of smaller capacity.

  • Most important, the larger drives are probably newer and, like most computer components, newer is going to be faster due to general technological development.
  • Bigger drives are also more dense, which means the head has to travel a shorter distance between data bits. This speeds up the throughput.
  • Bigger drives will have less data fragmentation, since there is more room to write files contiguously. This results in reduced seek time.

Drive and connection speed chart

The following chart lists sample speeds for hard drive devices. It can help you decide which external drive connection is right for you. Note that it’s only a rough guide. It is based on the general speeds of new hard drives of good brand-name quality.

Use this chart to help determine which parts of your storage configuration may be slowing you down. You can also use it to make sure that any new storage devices you buy will match the throughput of the connection type. (For instance, a high performance SSD would be wasted if it is on a slow FireWire 400 connection).

Figure 9This chart shows the connection speed of storage devices, connections types and network configurations, as measured in megabytes per second. These are typical speeds for maximum throughput when transferring big files. Small file transfer will be significantly slower, particularly for conventional hard drives.

Hard drive power supplies

Which power supply the drive will use depends on the case design. An internal drive added to a tower computer will use the computer’s power supply. This is tidier because you don’t have power cables running all over the place. It does tax the computer’s power supply, however, and that can lead to failure.

The power supply for single-drive external cases is typically a power brick that sits outside of the case. If you are going to use these, try always to buy the same brand so that you have swappable components to test if there is a problem.

The power supply for a multiple-drive enclosure is usually inside the case, and is a lot like the power supply inside your computer. If it fails, you can transfer the drives into another enclosure and keep working. (If the drives are in a RAID configuration, you’ll only want to transfer them to an exclosure with an identical RAID controller.)

Bus-powered drives

Portable drives with 2.5-inch disks inside often use the power in USB or FireWire cables to provide electricity to the drive. This is a real convenience for portable devices, but there are a few caveats. Some drives (particularly faster ones) require more current than is supplied by the port. In these cases, the drive will either not fully mount or might disappear when the power draw gets too large. Unfortunately, the only way to see if a drive works with your computer is to hook it up and give it a try.

There’s another note of caution that you should be aware of when using bus-powered drives. Too high a current draw can burn out the port that the drive is connected to. This seems to be typically associated with running multiple drives daisy chained off a laptop’s FireWire port. If you need to run more than one drive off a single port, you should buy one that will accept an external power adapter.

SMART status

Self-Monitoring Analysis and Reporting Technology (SMART) keeps track of status and error information for a drive and can be helpful in predicting drive failure. Most current computers can give you a pass/fail SMART status for internal drives, as well as for some eSATA-connected drives (if the eSATA port will support SMART data). You can also access the raw values, if you would like a more nuanced report on how well the drive is doing.

SMART data is not available for drives connected by FireWire or USB.


SMART Reporter panel
FIGURE 8 SMART Utility is a program that can read the raw SMART values from a drive and give you specific information about its status.

Hard drive volume configurations

Now that we know about drives and how they can physically be connected, we need to know about the logical configuration. Does each drive show up as a single volume, as multiple volume partitions, or do multiple drives show up as though they were a single drive?


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Last Updated September 22, 2015