IDE (Integrated Drive Electronics) is the name of a type of hard drive that uses the ATA (AT Attachment) interface. IDE's low-cost electronics and ATA's cheap parallel cabling makes it well suited for hooking up drives inside your case without breaking the bank. ATA is not well suited for hooking up drives externally and the signaling does not like to go further than about two feet in our experience. Longer cables are available, but we do not recommend them.

A single ATA channel can support up to two drives, a master and a slave drive. Usually you want to place a hard drive as a master and a slower device, such as a CD-ROM, as the slave. Since IDE can actually only access one drive per channel at a time, it is actually preferable to not have any slave drives. Nowadays, most systems come with two IDE channels integrated onto the motherboard, and we recommend users have their hard drive as the master of the first channel and their DVD or CD-ROM as the master of the second channel.

There are three main variants of IDE drives currently available: ATA/33, ATA/66, and ATA/100. The number refers to the peak bandwidth in megabytes of each variant. ATA/66 and ATA/100 require a special ATA/66/100 80-pin cable in order to run at their rated speed (that cable usually ships with any ATA/66/100 capable motherboard). Otherwise, they run at ATA/33 speeds. These three variants are also often referred to as UDMA. Though it isn't proper, you may hear UDMA, ATA, and IDE used interchangeably.

All IDE drives should work with all the variants of ATA. An ATA/100 drive should work fine with an ATA/33 controller, and an ATA/33 drive should work fine with an ATA/100 controller. The signaling will work at the speed of the slowest component. In both those cases, that would be ATA/33 speed, or 33MBps of maximum bandwidth. Sometimes you will run into IDE incompatibilities, where a certain drive doesn't work well with a certain cable, or where two drives from different manufacturers won't live happily on the same IDE channel, but those problems are relatively rare nowadays.

The actual real-world performance difference between ATA/33, 66 and 100 is usually quite small. There are no ATA/100 drive mechanisms available today that can use the entire 66MBps of ATA/66, and few that ever use up all of ATA/33's bandwidth. Only the drive's cache can make use of the added bandwidth, which yields some but not a lot of performance improvement.