from what i understand, this could be a power issue too.
do you have the double ended cable or just a single?
(single appears normal - standard usb plug on one end, some standard or proprietary end that goes into your device. the double will have an additional standard usb plug probably attached to the first by a short cable).
the double allows for more juice to power the external harddrive.
one came with my 2.5" enclosure. i've got an 80G drive in that one from an HP17" laptop that melted on me (i think it's a 5400). i've never had to use both connections. not sure if it would be faster with both. but i've had no issues either way.
i've also got a 3.5 inch enclosure with a 120G drive in it. however this puppy has it's own power supply.