I had the same problem trying to put an older drive (out of a gateway) into a new computer running XP Home. The new computer saw the drive, but would not assign it a drive letter. The problem was the older computer was use "GoBack". I fired up the old computer, uninstalled GoBack, and put the drive in the new computer, it saw it with no trouble then. My guess is that any software that screws around with the partition would give similar troubles.