How to Buy a Hard Drive

Typically, dual-drive RAID boxes offer the choice of RAID 0 or RAID 1. Multiple-drive direct-attached or network-attached boxes--some enclosures support up to five drive bays--generally complement those basic RAID levels with RAID 5 (parity--offering your best bet for data redundancy) or RAID 0+ 1.

You can also set up a RAID for your internal drives; however, your motherboard or add-in drive controller must support RAID.

All 3.5-inch, desktop-size internal SATA hard drives--as well as most current PATA drives--spin their disks at 7200 rpm. A handful of drives spin at 10,000 rpm; they are aimed at enthusiasts and enterprise users. Typically, the faster the disks spin, the faster the data is read and written--but the average buyer won't want to pay a price premium for a 10,000-rpm model.

Portable external drives have the biggest range in rotational speeds. Models are currently available in 4200-rpm, 5400-rpm, and 7200-rpm flavors. The most common of these is 5400 rpm. You'll see a difference in transfer speeds if you copy a lot of data--say, photos from a full 2GB memory card--to your hard drive, so keep a close eye on these specs, and beware of vendors that don't identify the drive's rotational speed.

Average seek speed, measured in milliseconds, refers to how fast, on average, drives can find a particular piece of data. This is a minor consideration: For most people, the effect of differences on this measure in everyday use is negligible. The exception is when a drive is called upon to assemble many small pieces of data scattered in different areas of the hard drive, such as when copying large folders full of many small files. Jumbo drives tend to have somewhat longer seek times.

Nearly all internal drives in new PCs use the SATA interface, which supports maximum transfer rates of either 150MB or 300MB per second. The drives with a 300MB-per-second maximum transfer rate cannot take advantage of their wider bandwidth in typical desktop use, though they shine in RAID combinations.

PATA drives, which support maximum transfer rates of either 100MB per second or 133MB per second, are still widely available. There's little advantage to one or the other; hard drives never sustain data-transfer rates approaching either maximum, though drives can sometimes push out of data at rates approaching the high-end speeds for brief bursts.

Both PATA and SATA interfaces are backward-compatible: For example, you can run an older ATA-33 drive on an ATA-133 bus or a 150MB-per-second SATA drive on a 300MB-per-second bus. But although PATA-to-SATA adapters are available, you should match a PATA drive to a PATA interface whenever possible, to get the best performance from it. SATA drives work with the SATA interface only. Inexpensive PCI Express and PCI add-in cards are available that let you add a SATA interface to a computer that lacks it; PATA and PATA/SATA models are available only for PCI.

Most external drives have a USB 2.0 interface or a dual USB/FireWire interface. Other interface configurations are dual USB2.0/FireWire 400; FireWire 800; and eSATA. USB 2.0 and FireWire 400 perform slightly slower than a comparable internal drive, but either is more than acceptable for auxiliary storage or backup. (A USB 2.0 drive will work with a USB 1.1 port, but its performance slows to an unacceptable level.)

FireWire 800 is quite fast, but this interface is relatively rare on desktop PCs. For high-performance external storage, go with an eSATA direct-attached drive. These drives are becoming increasingly popular; and while they don't offer the universal connectivity of a USB 2.0-enabled drive, they're as fast as an internal drive--and sometimes they include a USB connection, for good measure.

To install an eSATA drive, you'll need an open external port: Though eSATA drives come with a pass-through connector that provides an external port, you'll need an open internal SATA port to attach the connector to. If you don't have an open internal port but do have an available PCI slot, an inexpensive PCI add-in card can provide external SATA, USB 2.0, or FireWire ports for systems that lack them. Likewise, you can find PC Card adapters that add USB 2.0 and FireWire ports to a notebook, if your system is so old that it lacks these interfaces. However, FireWire and FireWire 800 use different connectors, so they are not cross-compatible.

Consumer-level NAS devices generally employ the TCP/IP protocol and a 10/100Base-T or gigabit ethernet connection to hook up to your network. As such, NAS devices are generally pokier than direct-attached external models. Look for NAS devices that provide USB 2.0 ports for accommodating additional hard drives so you can expand capacity or share attached printers across the network.