JAK:
I'm exhausting all ideas at this point, so please forgive me if this has already been considered.
You mentioned: "When I installed it in my other PC, I used the ide66 cable that was in that machine vs. moving the other one over, so I think I've eliminated the cable as an issue." Are you sure the other cable was cable select cable? or that you changed the jumper to master if it was not? I know, stupid question! but i've overlooked more obvious things before.
Also, did you check the CMOS settings? I'm not sure if you are still able to access the COMPAQ Computer Corporation Setup Utility. If you are, check the "IDE Option" under the Storage. The "IDE Drive Translation" should be set to Enable. I know, setting CMOS to the defaults should have taken care of this, but it's a long shot.
If, for some odd reason, it is set to Disable, use the arrow key to change it to Enable, press F10, then navigate to File, scroll down to "Save Changes and Exit."
When the PC reboots, keep pressing F10 till the word Setup appears, then navigate to Storage & scroll down to "IDE Devices" where it should show, for Primary Drive 0, at lease the size of the drive. If it states, "Not Installed," then i'm out of ideas & you may have to make funeral arrangements for the HDD.
Good luck!
Doc
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