What to look for in a new hard disk

The speed of your hard disk is a major factor of your system's overall performance, at least as much as its capacity. After all, the faster it's able to find data and transfer it, the quicker Windows will load, the faster your virtual memory will be, and the less time it will take to start applications and copy files.

Money is usually the deciding factor when choosing a drive, but with more money, people usually just opt for more gigabytes. If you want the best performance, though, consider these factors to be at least as important:

RPM (revolutions per minute) This is the speed at which the disk spins; higher numbers are faster. Cheap drives spin at 5,400 RPM, but you shouldn't settle for anything less than 7,200 RPM. 10,000 RPM (10k) drives are faster, but more expensive and harder to find. It's also worth noting that a larger-capacity drive can be faster than a smaller drive of the same RPM rating due to the higher data density.

Buffer (measured in megabytes)
The buffer is memory (RAM) installed in the drive's circuitry that allows it to accept data from your computer faster than it is able to physically write to the disk surface, and to read data from the disk surface faster when your PC isn't necessarily ready for it. A larger buffer is better; don't settle for less than 16-32 megabytes.

MTBF (measured in hours)
It doesn't matter how fast a drive is if it dies on you. The higher the MTBF-Mean Time Between Failures-the more reliable the drive is supposed to be. Of course, this isn't a guarantee, but rather merely an indicator of the market for which the drive was designed. Hard disks designed for servers tend to have much higher MTBF ratings than the low-end disks available on most computer store shelves.
If you're buying a drive for use in a DVR (Digital Video Recorder) or HTPC (Home Theater PC), it's also wise to seek out the quietest drive you can find. Some drives offer AAM (Automatic Acoustic Management) features, which let you quiet a drive at the expense of some performance. Although manufacturers typically offer very little in the way of useful, reliable noise data, you can usually cull pretty good feedback from HTPC discussion groups on the Web.

RAID-ready
If you want to set up RAID, you'll need two or more identical drives. In theory, there are no special requirements, but in practice, it's smart to stick with drives made for this purpose. For instance, Seagate makes two versions of most of their drives: the AS series (consumer grade) and the NS series (server grade). The latter of the two is more expensive, but is designed to cope better with the increased vibrations generated by a RAID array, and typically has a much higher MTBF than the lesser model. Plus, the firmware on these drives is more likely to play nicely with your RAID controller.

Interface
There's rarely any reason to buy anything but a SATA drive these days; if you can, get a drive with SATA3 and NCQ (Native Command Queuing).
PATA (a.k.a. ATA or IDE) is now totally obsolete. Even if your desktop PC has only IDE controllers, it's best to get a SATA drive and a cheap SATA PCI-E/PCI controller to go with it. The only time when you should consider an IDE is if you have an older laptop and you can't upgrade the interface.
Some weirdos may still prefer SCSI or SAS drives, but there are very few cases when that's preferable over SATA anymore. SCSI controllers are unreasonably expensive, as are SCSI drives; consider this option only if you absolutely need a 15k RPM drive.
If you're buying an external drive-which is great for backups, you may be tempted to get a USB drive or enclosure. While USB 2.0 is reasonably fast at 480 mbps, and Firewire 800 is slightly faster at 800 mbps, both of these standards will restrict the speed of your drive, and neither can be used to host a primary boot drive. For faster backups and less time spent transferring files, you'd be hard-pressed to beat eSATA (external SATA), which supports speeds up to 2,400 mbps. Most desktop PCs and some higher-end laptops include eSATA ports for this purpose, but if your PC doesn't have one, you can get an internal-to-external (SATA-to-eSATA) adapter cable for just a few dollars, or a standalone eSATA controller for not much more.