My syslog entries on Ubuntu x64 revealed lots of errors over the last month, specifically in relation to USB errors.
Jan 23 15:23:52 colossus kernel: [ 1806.970046] usb 1-1: reset high speed USB device using ehci_hcd and address 2
Jan 23 15:23:52 colossus kernel: [ 1807.131881] sd 4:0:0:0: [sdb] Unhandled error code
Jan 23 15:23:52 colossus kernel: [ 1807.131888] sd 4:0:0:0: [sdb] Result: hostbyte=DID_ERROR driverbyte=DRIVER_OK
Jan 23 15:23:52 colossus kernel: [ 1807.131894] end_request: I/O error, dev sdb, sector 205518871
Jan 23 15:24:53 colossus kernel: [ 1867.600085] usb 1-1: reset high speed USB device using ehci_hcd and address 2
These USB resets caused either of my two external HDDs to make the dreaded clicking noise, which made me suspect that either one or both of them might be faulty. Both drives were normally plugged into the back of the server on the motherboard itself, which has 4 ports available.
This time, due to work commitments, I had been ferrying my disk drive around and therefore found it more convenient to plug into the front mounted USB2 hub that I'd installed a couple of years ago.
After doing the usual rounds on the forums, quite a few posters were suggesting some physical damage to the hard disks whilst others suggested it could be an NTFS problem. A small percentage of posters suggested that using a hub may be the cause of the problems. After exhausting the first two options, I then plugged both HDDs into the motherboard USB ports and lo and behold, all was working fine again.