|Assuming that you haven't dropped the external drive since it last worked properly and there's nothing wrong with the external enclosure's circuits.... |
Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1:
It's about those things in Windows but similar applies to any operating system.
If your external enclosure is damaged or defective , it can be replaced cheaply.
If your hard drive is damaged or is failing there's not much you can do
The hard drive that's inside the external drive can be tested with hard drive manufacturer's diagnostics by connecting it to a different computer, one way or another.
Test your hard drive with hard drive manufacturer's diagnostics.
Seagate's SeaTools will test (almost) any brand of hard drive.
Do the long test.
The Dos bootable versions of SeaTools can test the hard drive when Windows will not load properly, or even when the drive has no data on it.
The Windows version of SeaTools must be used when the drive is in an external enclosure.
It the drive itself passes the test, any data problems on the drive can be fixed one way or another.