Hey Lululee,
  I'm curious, did the "new" drive hook up as a parallel drive (40 pins, big thick cable) ? or a serial drive with the newer thin cable??? Are they on the same cable (master/slave setup) Some more
info such as original drive Type/Size/Speed or Rpm and XP or Vista OS would help. I think my "My
Book" is an older 7200 rpm Parallel drive. They make kits I think to convert old drives into USB drives. More info would be helpful. Have you double checked any jumper settings on the "new"
drive??? I'll try to check back later but more info would be helpful, even the brand name unless
you built this yourself. If so you may wanna check the manual on your mother board. Quite a good idea on your part doing this, kudos for that. (Were new power supplies that expensive, any
options on a generic power supply, some thing with different voltages and polarity at the drive
side of the supply??? Later......Holy Diver

Oh, you could always back up and punt, meaning disconnect the "new" one and make sure
everything is OK as originally thought. Just to be sure. You never know with Poser.....