How? I replaced the 500GB hard drive with a new one. Put the T1000 in 'maintenance mode' , selected 'USB firmware update - delete recordings', follow the on-screen instructions and let it do its stuff. After setting up, it is back to as good as new.
No idea what is wrong with the old hard drive - I conneted it to a Linux Mint PC and did a thorough disk check and it came up OK, no errors. I can even play most of the recording on the disk on the Linux Mint PC. It is possible that there is a corrupt system file on the disk that is caused the problem - though I would have thought that the operating system in the T1000 (Linux?) should have detected and repaired it.
I will try and transfer my recordings from the old disk to the new disk - see how easy or difficult that is to do. Watch this space!!!