Today I left it running and when I came back to it the TestDisk window had disappeared. I don't know if it crashed or what. There was nothing in Windows Event Viewer to suggest it had crashed.
It was at about 84% and then half an hour later, gone.
Anyway I went to the log it had created and wonder if a) it had in fact finished and b) if it's possible to interpret the log to allow me to fix my issue.
My issue is that I was reformatting an external hard disk, changing the file system on a blank partition from NTFS to HFS+ and suddenly the other partition which actually had data on it vanished. And no, I didn't accidentally choose the wrong partition to format. I was beyond careful about double and triple checking which partition I had selected to format
Any help much appreciated