Connected drive externally to another computer and could see the drive but not do anything with it .... error message was "F:/ is not accessible. The file or directory is corrupted and unreadable".
Ran Check Disk and it immediately started finding a number of errors and correcting them. When finished, I could access the drive as normal. To be completely 'restored', ran all 5 phases of XP's Error Checking, invoking 'fix file system' and 'recover bad sectors' .... took about three hours.
Reinstalled drive and it's running fine.
Can any body offer reason/s why such a major corruption may have happend in the first place? The drive is 4 years old.