I hope that in my effort to reduce all the confusion surrounding real and unofficial
IDE/ATA standards, that I have not made you even more confused. 
There are a lot of
buzzwords being tossed around, and the marketing people are hard at work introducing new
ones every year. ;^)
Fortunately, the technology itself is pretty easy to use, even if the labels given to
it often stink. So one useful way of dealing with all the standards and labels is simply
to ignore them! Look past the hype, and focus on what the drive's actual capabilities are.
If you want to really understand what a drive can do and what it supports, you should look
at its specification sheet and see what features and transfer modes it is designed to use.
Ignore labels like "EIDE" or "Ultra ATA/whatever" and find out what
modes and functions the drive supports. Getting the real scoop on the drive means you
don't need to worry about the pretty stickers slapped all over the box, or whatever the
manufacturer is trying to claim.
It's also important to realize that despite all of the various names and flavors, to
some extent IDE/ATA is IDE/ATA. Especially since the late 1990s, virtually all IDE/ATA
drives and controllers will work together with a minimum of fuss. The folks who create the
IDE/ATA standards always ensure backwards compatibility. This means that a drive
corresponding to a newer standard will still work on an IDE channel in an older PC.
Similarly, older drives will work on newer systems, in most cases. When older and newer
hardware are mixed, the newer hardware will just run at whatever the maximum speed is of
the older hardware.
For example, suppose you want to use an older hard disk that does not support Ultra
DMA, on a newer PC with Ultra DMA support. This will work fine, but the drive won't run at
Ultra DMA speed. As a second example, suppose you want to install a new hard drive that
runs Ultra DMA/100 on an older PC with a hard disk controller only supporting Ultra
DMA/33. This will also work, but the drive's throughput will be limited to 33 MB/s.
Note: There can be issues
with using some newer drives on some older systems, if they come enabled to run at
higher-speed Ultra DMA modes by default. A utility may be needed to change the default
transfer mode of the drive. See here for more details.
Similarly, using new drives on older systems may cause problems
related to the larger sizes of new drives (which isn't an interface issue).
One issue that many people have when upgrading an older system with a newer hard disk
is that the older system may not support the highest transfer rate supported by the drive
(as in the second example I just gave). This often causes great consternation and anguish,
because the hard disk manufacturers hype interface transfer speeds to the high heavens. In
fact, running an Ultra DMA/100 drive on an Ultra DMA/66-capable controller will produce no
noticeable difference in performance compared to running it on an "Ultra ATA/100
controller". Even a 33 MB/s "regular" Ultra ATA channel will not result in
a huge performance hit. For a full explanation of the reasons why, see this page.
Next: IDE/ATA
Transfer Modes and Protocols
