Problem with Macs which is actually a beni is they read and write just about any format, DOS, Fat32, Unix etc... so it may not always pop as unitialized.
Use Disc Utitlites.
May also be a jumper issue. All of my removable 2.5's use no jumpers at all.
DId you get a USB enclosure or Firewire. I have both and the Firewire is so much better then the USB versions.
I find the USB cases need 2 usb ports to supply enough power and then sometimes it's a pain to get the disc to mount.
Get one of these cases,
http://macally.com/spec/firewire/storage/25enclosure.html
Scott