Inside it's a totally common IDE or SATA drive (more likely IDE..depending on the model)
So, like already mentioned you either put it inside the comp or get a new enclosure or option #3 get some form of IDE 2USB adaptor.
When my 1st. external gave up after a year (power supply gave out), I just bought this enclosure
for $20.00 incl external power supply (because my drive was 3.5"...too big for power through USB)
It runs fine ever since.