QT files suddenly unreadable and off-line

Trying to help a friend to capture his DV Pal footage with FCP 4, we obtained 3 QT files that were readable either in FCP from the browser or directly from the QT file through QT player. After a few hours work those 3 QT files became suddenly off-line in the browser and unreadable with QT player. I tried to drag them from the capture scratch folder into the browser and got the following message : FCP does not recognize file type. Do I need to recapture. I never had this problem with FCP captures, is there something to do to prevent this to happen again ? Many thanks for your help

Barberine

But that doesn't explain why quicktime can't read the files either. I have the same problem here, I have imported two days worth of old vhs footage using fcp only as an import device to create QTs on a 1TB USB drive (I know firewire would be preferred, but this is what the client provided, and I won't be editing from it). I have constantly checked the files in fcp as I have imported.
Now the files cannot be read by fcp or QT. I have taken the drive to another computer, with the same results. We are talking hours of footage here, what can be done?

How did you bring the VHS media, which is analog, into FCP which reads digital files, not analog?

--ken