Hard Drive 101

How does a hard drive work?

Figure 1 shows the inside of a hard disk drive. 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 the each magnet can be “up” or “down,” which indicates whether the bit is a 1 or a 0. The read/write head is like a record tone arm, which 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 SCSI 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.

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 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

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 of this 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 four 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 capacity

The capacity of a hard drive is 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 are not calculated in the same way that your operating system calculates data sizes. For instance, a drive sold as “500 GB” actually only contains 465 GB (actually, the 500 number is Gibibytes, and the 465 number is Gigabytes. Aren’t you glad you asked?)

We generally suggest that it’s better to get the largest capacity 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.

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 any offline backup drives. 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 spin at 4200, 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 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.

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.

ATA/IDE Cable

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’re buying new enclosures, we suggest staying away from PATA in favor of SATA.

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.

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

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), 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 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.

You can also purchase a freestanding enclosure and an internal drive and put them together, like the one pictured at right. 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.

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 three 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.

Figure 6shows an external drive with all the most common interfaces.

USB

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 is maxed out at a theoretical 60 megabytes per second. Due to the USB drivers in the Mac OS, USB is considerably slower on Apple machines. A USB 3.0 version is in the works and promises to offer a tenfold increase in theoretical performance. USB connectors can supply bus power to attached devices.

FireWire

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 also can offer bus power to run external drives if the FireWire port is a six-pin port. (Many PCs only offer four-pin ports.)

eSata

eSATA is a configuration that creates a SATA connection in an external enclosure. It’s 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 does not have the capability to bus-power hard drives so you must use an external power source.

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.

iSCSI

iSCSI is a connection method that uses existing Ethernet hardware to attach the storage to the computer. An iSCSI device can be directly attached 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 megabytes per second.

Note that iSCSI needs "initiator" software that manages the connection. Some devices, such as the DroboPro shown in Figure 7 include this software. Other iSCSI device manufacturers suggest you purchase separate iSCSI Initiator software.

Figure 7shows the connectors on a DroboPro unit. From left to right they are USB, Firewire 800 and iSCSI.

SCSI/SAS

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 II, in the neighborhood of 300 megabytes per second.

Fibre Channel

Fibre Channel is a technology that has migrated from supercomputers down to enterprise-level storage (big companies) and will likely be available for small office, home office (SOHO) users eventually. 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 400 megabytes per second.

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 (watch out for an SPF).

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

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 hard drive and can be helpful in predicting drive failure. Most current computers can give you a pass/fail SMART status for internal drives. You can also access the raw values, if you would like a more nuanced report on how well the drive is doing.
Read more in the data validation section

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.

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