Hard Disk Monitoring

I ran the surface test "read + write test + read (deep test)" on the external hard drive. A message appeared in the middle of the test, namely "Error 121 The semaphore timeout period has expired". What has happened? What does it mean? And what should I do?

It means that the hard disk encountered a serious error and can't recover from that - and as the disk drive is not responding at all, the system disconnected it due to timeout.

This can happen with some (usually older, simpler) USB adapters/docking stations as some of them may not have good error-handling. You may try the test again as hopefully after some retries, the problems could be fixed and the disk can advance.

If the result is always similar, then if possible, try to remove the disk drive from the external enclosure and try with different USB adapter/docking station or try to connect directly to a SATA port on the motherboard, as the different connection / adapter may work differently.