IDE refers to a particular type of hard drive interface. The drive has the hard disk controller on it, and a card only provides a stripped down portion of the 16-bit ISA connector for the ribbon cable. Since the IBM PC AT was the first computer to use a 16-bit ISA connector (originally referred to as the AT bus), thats where the term comes from. (Compare the 8-bit ISA connector, originally referred to as the XT bus, from the IBM PC/XT). So in almost case the two terms are synonymous. However, if your hard disk requires a different type of 16-bit ISA interface card, like SCSI or MFM/RLL, that could still be called an AT bus hard disk.

A hardcard also comes to mind.

Well for AT compatible hard disks the Master/Slave handling was not standardized and worked only reliable with HDDs from the same manufacturer e.g. Seagate.
AT-A compatible HDDs usually work well as Master/Slave when jumpered right.

It is written on the drive if it is AT or AT-A compatible.

There are a few different types of IDE. Originally there was 8-bit XT-IDE (XTA) and 16-bit AT-IDE (ATA). AT-IDE had the specification extended a couple of times...."EIDE" was one of them. I don't know for certain, but my feeling is that the specification for EIDE must have been pretty loose like SVGA.

XT-IDE died out pretty quickly for the obvious reasons. So now when you hear that term IDE it is usually taken to mean ATA and vice versa.

Anonymous Coward wrote:

XT-IDE died out pretty quickly for the obvious reasons.

Upper limit of 40MB hard disks were among the primary reasons, since it was based off the old IBM/Xebec hard drive controller. By the time it was introduced 16-bit ISA slots were almost universally standard. But it has been resurrected (in a way) in the modern XT-IDE project(s).