You can reformat the HD but you'd lose your existing recordings. I no longer have sky + but I did do this once although I don't think my box was an amstrad.
1. Press Services
2. Press 4
3. Press 01 then Select
4. Press 8 (Housekeeping)
Wait a few minutes and it should be OK. If not you're on the phone to Sky who'll tell you it's £65 for an engineer. (That goes down to £0 if you threaten to leave)
Just found a more detailed method:
http://www.howtomendit.com/answers.php?id=94945