Jack,

I think it will be difficult to give an immediate clue to the problem. You have not mentioned the OS on your present computer and the OS on which the HDD was "originally formatted".

The simplest possibility is that this troublesome HDD is formatted in NTFS on an XP machine and yours (and the 3 other PC's)is running 98 which cannot read NTFS.

The other "good boy" HDD may be in FAT 32 format which is readable in both XP and 98.

A little more input would help us to give other possibilities....