Found the problem - it was caused by a file "shredder" - eraser.exe - although not it's fault. There are a few of these now - designed to obliterate confidential files that have been deleted but are often left intact on the drive but no longer in the file system. I had not used it much and set it to erase all "empty" space on the drive rather than individual files.
The program works not by erasing the data but by actually writing something over the unused space. So it keeps writing until it fills the drive - at which pooint windows says it is short of disc space and deletes the "non-esse