Hard Drive works internally, but not externally.

Hello everyone, I've got a Seagate Momentus hard drive taken from a laptop that I intend to use as an external storage device. When plugged directly into my laptop's hard drive slot, it works fine. When I try to use it externally with an enclosure, it is not recognised (on the same computer from which it was taken) by either Windows Explorer or Disk Management (yes I did a rescan). ...