The drive being visible has multiple parts:
1) The USB interface must be working in order for the computer to recognize that there is a device at all.
2) The "driver" board on the disk must be somewhat working in order for the system to realize that the device is a disk and exists.
3) The actual spinning rust (called a disk) must be readable in order to store or retrieve files.
I think you may be having a problem with the USB interface and/or the "driver" board of the disc itself. Without seeing the device and poking around a bit, I can't be sure.
Dan