External USB hard drives are either PATA or SATA interface hard drives connected to a USB adapter circuit. Open the enclosure and you should see whether it's PATA or SATA. If you have a spare slot on one of your computers, along with an appropriate hard drive cable, you can connect the hard drive directly, via the cable, after you've taken the drive out of the USB enclosure. That will help you determine whether the enclosure's USB adapter has gone bad.