Not sure where to exactly tell you to go from here. It sounds like you have everything the way that it should work now but it isn't...
I'd almost suggest trying a different HD case, but according to the copy/paset from your original statement, it sounds like you have already tried different cases.
{I bought a number of cases that are able to accept what were internal HD's and use them instead as external devices connected by 2.0 USB connections.
The 2 computers i am eneavoring to use the device on are both running WinXP Pro SP2.Both computers have front and rear 2,0 USB ports.
I have tried 2 computers and 2 different inernal HD's.}
Just out of curiousity are the cases all the same make, or different models, have you contacted the support from the case MFR to see if they can assist. At this point I would almost suggest that being that you have somewhat similar results to where the drives wont talk two 2 computers that otherwise talk fine with other USB devices that its a case issues IDE/USB translation board, since the configuration is correct now for the IDE drives themselves.
Only other issues I've seen with USB Ext drives are where people add extensions to the USB cables and exceed 18 feet, and where Windows 98 has issues supporting USB Ext HD's, and a CD supplied with the external drive has to be installed to support communications.
The other thing done wrong is that people introduce the Ext HD in an uninitialized state. Its best to turn on the USB EXT HD before connecting it to the USB port. Then its already initialized against the internat IDE/USB translation board. Then connect to USB port and you would then see on the system tray that a drive was found, and then it should tell you that the drive is now ready for use.
Good Luck...
Dave