Thanks for the info guys... it seems no matter what I do, whether to format, diskpart, chkdsk etc; whether from Windows UI or cmd.exe, comes up with "could not perform operation due to an I/O error". So I assume the disk is buggered (surprised me how suddenly this happened, without Windows warnings that the drive was about to fail) and wouldn't be trusting it again. Fortunately, it was only a backup for the storage D drive so I haven't lost anything...
I'll take the advice & chuck in a new large capacity drive (probably a WD green 2 or 3 TB)... the WD's have been pretty good to me.
Cheerz all
