|A second look finds you specify a SCSI hard drive.|
I would think similar to the following applies to your case as well.
Older mboards / bioses will NOT detect an IDE hard drive properly if it's set to Slave and by itself on a data cable. You MAY get strange symptoms in that situation with such an older mboard / bios, and in that case it doesn't necessarily indicate there's anything wrong with the drive.
Newer mboards / bioses will detect the hard drive anyway in that situation, but it's not recommended you do that.
It is common to un-intentionally damage IDE data cables, especially while removing them - the 80 wire ones are more likely to be damaged. What usually happens is the cable is ripped at either edge and the wires there are either damaged or severed, often right at a connector or under it's cable clamp there, where it's hard to see - if a wire is severed but it's ends are touching, the connection is intermittant, rather than being reliable.
Another common thing is for the data cable to be separated from the connector contacts a bit after you have removed a cable - there should be no gap between the data cable and the connector - if there is press the cable against the connector to eliminate the gap.
80 wire data cables are also easily damaged at either edge if the cable is sharply creased at a fold in the cable.
Try another data cable if in doubt.
Once you're sure the data cable is okay, if the drive is by itself on a data cable, set the drive to master or cable select, and...
Check your hard drive with the manufacturer's diagnostics.
See the latter part of response 1 in this:
(thanks to Dan Penny for this link:)
Hard Drive Diagnostics Tools and Utilities
If you don't have a floppy drive, you can get a CD image diagnostic utility from most hard drive manufacturer's web sites, but obviously you would need to make a burned CD, preferably a CD-R for best compatibility, on another computer if you need to.
If the hard drive itself tests okay, any data problems found can be fixed, one way or another.
Note that some IDE hard drives, e.g. many Western Digital models, have two ways they can be set to Master - e.g. Master (single), for when it is by itself on a data cable, and Master (with slave), for when it is on a data cable with another drive set to Slave. That must be correct for the situation, otherwise the drive(s) will probably not be correctly detected.