Are you asking about powering a hard drive with a second PSU? There's no issue with that and it's not a bad idea if you need the harddrive in that PC. It will just look a little corkey.
Thanks for your reply's the real story is that I have three IBM xServer 345s IDE but the company I bought them from sent my SCSI and I am too cheap to buy SCSI disks and there are no molex power connectors in the machine
Ok so from these opinions I'm going to assume that its not that bad of an idea...