|Faulty computer power supplies are common.|
Check the current voltages in your bios Setup - if the +5v reading is more than 10% lower than 5v (< 4.5v) , your USB, and firewire if you have that, won't work properly, and you may not get the normal data trasfer rate between the external drive and the computer. If that's the case, or if any voltage (+3.3, +5, +12) is more than 10% high or low, you need to replace your computer PS.
If that's okay, it's not unheard of for the external power supply for an external drive enclosure to become faulty. If it is faulty your hard drive may not work properly, and/or it won't be able to transfer data as fast as it should, but there is probably nothing wrong with it. If you run a hard drive diagnostic on the drive and the enclosure circuits or it's PS has a problem, the results of the test might not be valid.
If you have a spare internal connector,IDE or SATA, the hard drive can connect to (if you don't temporarily disconnect a CD drive), remove the drive from the enclosure, jumper the drive appropriately if you need to, try it in your case.
If it works fine, you need to get another external power supply for the external case, or replace the external enclosure.
If it is still slow,
Check your hard drive that was in the external case.
See the latter part of response 1 in this: