Hard Drive 101

Hard drives are the best choice to store a digital image collection (and most other kinds of digital files). This lesson outlines the most important points you will need to know to build and manage your hard drive storage.

How does a hard drive work?
Hard drive sizes
Hard drive capacity
Hard drive rotation speeds
Hard drive interface
Solid state drives
Hard drive enclosures
External hard drive connections
Hard drive power supplies
Hard drive volume structure

How does a hard drive work?

drive platter

Figure 1 shows the inside of a hard disk drive. Inside the drive is a disk platter that spins really fast. While the disk platter looks like a mirror, it is actually composed of up to trillions of tiny magnets that remember the 1's and 0's of your digital files. As you save files to disk, the digits are written, and as you access files, the numbers are read out and sent on to the computer. 

In addition to the platter and the read/write head, each drive has controller circuitry, as well as an interface to connect with.

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 1 There are 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

Hard drive capacity

The capacity of a hard drive is the amount of data it can hold. These days, capacity is measured in gigabytes (GB) or terabytes (TB).

I generally suggest that it is better to get the largest capacity you are likely to need, at least for the next 12 months (if you are on a RAID system, you will want a longer time frame—maybe 2 years—due to the complexity of upgrade). Running fewer drives saves on space, power draw and heat generation. It is also easier to manage your drives if there are fewer of them.

If you have several smaller capacity drives, itis probably time to think about combining the contents onto a single larger drive. Any 3.5 inch drives that are 250GB or less in capacity are probably getting old enough to be retired anyway.

Hard drive rotation speeds

As part of its specifications, each hard drive lists a rotation speed, measured in RPMs. The faster the drive, the faster the it can read and write data. Since reading and writing to disk is often a computing bottleneck, this can be an easy place to speed things up in your workflow.

2.5-inch consumer drives spin at 4200, 5400, and 7200 RPMs. If you are using a laptop as a digital imaging computer, you will want the internal drive spin at 7200RPMs. Downloading files, working with Lightroom, opening in Photoshop, saving in Photoshop will all be faster with a faster drive. Slower drives can be fine as a laptop backup drive, since the speed here is generally less critical. 

3.5-inch drives generally come in 5400,  7200, 10,000, and 15,000 RPM models. 7200 RPM models are good all-purpose drives for working files. The slower drives can be a good choice for archive storage if you do not need to access the files a lot. The slower drives also generally use less electrical power and generate less heat. 

Hard drive interface

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 most typical are ATA/IDE and SATA. SATA has been the standard since about 2007. 


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 now, 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 are buying new enclosures, I suggest staying away from PATA in favor of SATA.


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.

Solid state drives


A new kind of storage device for computers is just beginning to show up in the marketplace. Instead of spinning disks, flash memory is being used as primary storage. Your camera's memory card is flash memory storage. This type of device, called a solid-state drive (SSD), offers some important advantages, including extra speed, shock resistance, greatly reduced power draw and potentially greater reliability since they have no moving parts. At the time writing, SSDs are only available in very limited capacity (less than half that of spinning disk) and they command a relatively high price— more than 4 times the cost-per-gigabyte of 2.5-inch disk.

This is an area of fast growth, and due to its advantages, I expect that SSD will be a pretty commonplace device before too long in any level of portable computing. They will also be commonly used for boot drives and scratch disk in desktop environments, where greater speed makes a big difference.

Hard drive enclosures

Now that I have gone over some characteristics of hard drive mechanisms, you should 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 are not as easy to swap out as external drives.

Single drive enclosures

If you do not 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), USB2, 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 2 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.

You can also purchase a freestanding enclosure and an internal drive and put them together, like the one pictured on the right. I like this option because it offers more control over the components and because I can reuse the case when I 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 is 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 favour.

4 drive externals

Figure 2 A 4-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 connections

The hard drive mechanism has its internal interface (PATA or  SATA), 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 3 principal ones in use, and a few additional ones that are used in high-end systems. Figure 3 shows a drive that has the 3 most common connection types.

external Hard Drive Interfaces

Figure 3An 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 or USB 3 (stay away from USB 1 because of its slow speeds) is a good way to connect external drives.

On USB 2, data throughput is usually about 25 Megabytes per second. Due to the USB drivers in the Mac OS, USB is considerably slower on Apple machines.

USB 3.0 is now available and in can provide a transfer rate of 400 Megabytes per second..


FireWire 400 and FireWire 800 (also known as IEEE1394 and IEE1394b) are standard on Apple computers, and are found on some PCs. Firewire provides 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 also can offer bus power to run external drives if the FireWire port is a six-pin port. (Many PCs only offer 4-pin ports.)


eSATA is a configuration that creates a SATA connection in an external enclosure. It is a fast and stable connection, offering up to 150 or 300 megabytes per second. eSATA is beginning to show up on computers as a built-in external interface. You can add eSATA to older computers by means of an expansion card. 

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 is really hot-swappable.

Hard drive power supplies

Hard drives require power to operate. The final item I will outline is the various ways that a drive can get its electrical power.

Internal drives

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

External drives

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 to always buy the same brand so that you have swappable components if the power supply goes bad. (This actually happens pretty often).

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 often replace it with a generic one from a local computer store.

Bus-powerd drives

Portable drives with 2.5-inch disks often get their electricity from the USB or FireWire cables. This is a real convenience for portable devices, but there are a few caveats. Some drives (particularly faster ones) require more current than may 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 is 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.

Hard drive volume structure

When you set up a hard disk, you have some options for the volume structure. The illustration in Figure 4 shows how this appears.

  • A drive may formatted as a single volume. Each disk can appear as if it is one single hard drive. This is the simplest and most common structure, and it is called JBOD, meaning Just a Bunch of Disks.
  • Drives can also be 'spanned'. In this arrangement, several different drives appear as though they are one single drive. By combining disks, you can get greater speed for tasks like video editing. You can also span drives to get extra protection. By writing each bit of data to two different drives in an array, the data may be able to survive the loss of a single drive. This is typically called a RAID structure.

Figure 4 This 4-bay hard drive enclosure can have several different volume structures. JBOD is on the left, where each disk in the enclosure appears to be one drive. On the right, the disks have been spanned using RAID technology. The can add speed, security or both


In a JBOD system, each hard drive attached to the system appears as a single drive. It is the simplest arrangement for multi-disk storage, and the easiest to configure, upgrade, and repair. JBOD is a good configuration for photographers to use as primary storage for a couple reasons.

  • It is simple to purchase and set up. Making an informed choice on the best RAID implementation, on the other hand, is not something that should be done without a good understanding of the hardware and software under the hood. Few photographers are well equipped to make this decision properly.
  • It is generally much quicker and easier to upgrade capacity with JBOD. A traditional RAID setup must be completely rebuilt (all data moved off, all new drives installed) to upgrade storage size. This makes the upgrade process a difficult, time-consuming, and possibly dangerous process. With JBOD, you simply add another drive or replace one drive with another drive of larger capacity.
  • It is the most economical configuration.
  • Although RAID is theoretically safer, too many people have lost their entire RAID systems for me to be convinced that it is practically safer, at least with consumer-level RAID.


RAID is an acronym that stands for Redundant Array of Independent Disks. In a RAID setup, several drives are configured together to act as a single volume. There are different flavors of RAID, and each one spans the drives in a different way. 

  • The data can be stripped, meaning that files are broken up into pieces and written to several drives simultaneously.  This provides a large capacity and faster throughput, but it makes the whole arrangement more fragile. 
  • The data can be mirrored. In this case, an exact copy is made on a second drive. You do not get any increase in speed or capacity, but you do get extra security. 
  • The data can also be written to an additional drive in a coded way called parity. This provides for making a larger drive volume that also has extra security.
  • In general, I suggest that RAID should only be implemented by someone who knows that they are doing, and only when there is a real need to use it. I've seen a lot of RAID systems totally fail, often because the person who put it together did not really know how to purchase or maintain it optimally.

Read more about RAID at dpBestflow.org


The previous section might have made you nervous about getting a RAID setup. There is a consumer-level option to consider if you’re looking for a fault-tolerant storage device: Drobo. Drobo’s goal is to offer many of the advantages of RAID with some significant improvements, all in a device that is a simple storage appliance.

At first glance, Drobo acts like a RAID—it spans up to four drives to make one logical drive. It offers the same kind of parity protection you get with RAID 5, where the data can survive the loss of any single drive (or, if the drives are large enough, the loss of any two drives, like in RAID 6).

However, Drobo offers some things RAID does not. It can intelligently make use of drives of different sizes (unlike RAID, where all drives are chopped down to the size of the smallest drive). When you need to upgrade the capacity of your Drobo system, just take out the smallest drive and replace it with a larger one. The data will be swapped around, making the best use of the new drive space.

Drobo also has some very simple 'gas gauge' lights that show you how full it is. As the disks get full, it will show you on the front of the box itself, as well as in the Drobo Dashboard software that comes with the unit. It connects to the computer by USB2 or FireWire 800 connection, and offers an add-on Ethernet connection for use as a NAS drive.

Drobo also has some intelligent power consumption features. It goes into a very low-power-draw sleep when not connected or not used, then wakes up when it’s needed. And because it is a standardized item, the controller card is likely to be available in the future (in the event that the one in your unit dies and you need to retrieve your data).


DroboPro is the enterprise-class storage version of Drobo. DroboPro adds support for up to 8 drives, and includes a faster processor for better throughput. DroboPro also makes use of the iSCSI connection protocol. It is suitable for high quality video editing, and has all the ease of implementation of the smaller version.


Figure 5 The DroboPro can span up to 8 drives for up to 16 TB of storage