render files going offline.....

Here's a funny one- got two identical Mac intels (quads 2.66) with 4gig ram,ox 10.5.6 qt 7.6 and fcp 6.0.5

Both running projects from G-RAIDs through FW800, not great but it works.
Now one project, fully rendered and finished is moved to the other machine to have tops and tails made.

The sequences are opened as and when they are need to be looked through but on the 2nd time of opening they lose 3/4 of their render files.
The drive was the only scratch disk for the entire project and no render files exist anywhere else but on this drive.
All the sequences were qt'd but it'd be handy to have the fully rendered sequences just in case.

Can anyone explain why render files would be dumped unexpectedly almost before our eyes? I think we have time to re-render in which case i'll dump the old render files and make new ones....

Any advice much apprciated....