Hard drives do not consume all that much power .. I think like 10 to 30 watts max depending on the drive. At one time (a few years back) I put a minitower On Top of my midtower so that I would have room for 5 more drives. Ended up using 10gb IDE, 4gb IDE, 9GB scsi (full hight 5.25in),9GB scsi (full hight 5.25in), 2gb SCSI, 2gb scsi, 1gb scsi, and When they came out I added a 20GB IDE to the mix (was using a 2x IDE cdr with an 8x cdrom) A Celeron 300a on a BH6. Everything would run off of a 250 watt PS, but i thought that i was pushing it at the time, so i hooked 4 drives up to a seperate 150 watt AT power supply, and let the Main PS power the rest.