What do you mean by “is working”? Just because the lights come on doesn’t mean it’s working.
For example, I have several external hard drives – two in particular (both Western Digital 500GB) went bad on me – I took the actual hard drives out of their enclosures and put them in a new Rosewill enclosure and tried to hook them up to my system. One did just fine, thereby letting me know it was something wrong with the interface (HD is SATA, external connection was USB 2.0). The other just made clicking sounds, which is a telltale sign of a head crash, making the drive npn-usable (priced out repairs – it would cost $$$$$$$$ to repair it – something like $1000 – as it needs to have the platters transferred to a new hard drive casing with new heads and all done in a clean room).
So the questions I have for you are:
1) Is it making any sounds on powerup? If so, check out this page: http://datacent.com/hard_drive_sounds.php
2) What messages are you getting (if any) and if none, what is the behavior when you try to attach it?
