This link is quite old, but it might help.
did you try connecting it to another computer?

Thank you all - it's been a busy weekend, and I'm planning to give the system a complete going over on Monday - including trying the suspect drive/cable on MrsBG's iMac to eliminate a couple of possibilities. Just hope I don't eliminate her iMac too ...

Meantime I've done a lot of maintenance and file sorting out elsewhere, just in case!