It could be the cable, but a higher chance that the drive has physical issues. Does it have any valuable data on it that isn't backed up?
+1
I suspect my title is what happens next but let me elaborate on the
RecoveryForce's post.
Cables first, then depending on if you need to recover the data -
proceed with caution
Since typically a backup drive has original data somewhere else, the first step might be to get another backup drive and continue backing up your important data.
The problem may well be the SMART (Self-Monitoring, Analysis and Reporting Technology) has decided not to recognize the disk. I sometimes think SMART was designed to sell more disks.
Since it is not in warranty take it apart carefully, there is a typical (
non-usb, hook up to a desktop) hard-drive in there, separate it from the controller board and test the drive in a machine; My hardware friend has a test bench where he can test the drive easily,
(I let him do the hardware work for me and I write him code in return) but just run the wires outside the your desktop computer and see if it recognizes the drive. If it does, the culprit may be the controller.
If you need your data proceed with caution, if not try to reformat the drive. SMART isn't always correct.
Just my thoughts, last time I messed with a hard drive was to manual reset the boot sector to DOS from CP/M or something my friend had tried and after reformatting, could use the drive in his DOS machine. Ancient history, nowadays that's what techs are for at work and your wallet is for at home.