Capacity: When shopping for a new hard drive, compare capacity or amount of available storage space. As mentioned previously, you want the largest hard drive you can afford. Hard drives range from 500MB to 10GB or more.

Access time: The lower the better. In other words, you want a hard drive that can do a read/write operation in the shortest amount of time. Access times usually range from 10-20 milliseconds.

MTBF: This number indicates how long your hard drive will last and is usually listed in hours. The higher the number the longer the hard drive will last.

Disk cache: Sometimes called a buffer. All hard drives do not have disk cache, but if you can find it, your hard drive will work faster and more efficiently.

Cost per MB: This is an important indicator in comparing hard drives of different capacity. Usually, a larger capacity hard drive will have a lower cost per MB than a lower capacity hard drive. So you usually get a better deal buying a larger capacity hard drive, even though it might cost you more.

IDE or SCSI: You should usually purchase a hard drive to match your PC's existing configuration. For example, if your PC currently has an IDE hard drive, your new hard drive should also be IDE. If you want to change to a new drive type, like SCSI, you must buy a new hard drive controller.

Bays: All of your drive bays used up? You can buy an external hard drive that connects to your parallel or SCSI port. You also can buy a hard card which is a hard drive on an expansion card that installs into an expansion slot. These types of drives cost more.

Power supply: You may need to upgrade your power supply if you are adding a hard drive to a PC that has never been upgraded before.