"Unknown File Type" -- Corruption Problem?

[Karen Robbins]

Hi Framers,

I've read a few threads on this in the archives but didn't get how 
they could help solve this problem.

Using FM 9.0p255 on a virtual PC, Windows 7, via Parallels.

I have a set of well-established files on an external HD that I take 
between home and office to work on the files regularly. Yesterday, 
one file generated the dreaded "unknown file type" error and nothing 
seemed to open it. Because I'd had a modem power supply go bad around 
the same time as I last worked on this file (at home), I accepted the 
corruption and was able to restore the content from other sources 
into a new file.

Today, a different file that was fine yesterday afternoon at home now 
generates the same "unknown file type" error at the office. And this 
file I don't relish reconstructing. The result is the same whether I 
double-click from the Mac, open from the Windows desktop or folder, 
or choose File>Open in Frame. From the Mac side it looks like the 
permissions are OK; don't know how to check that on the PC side.

I'm concerned that the problem is random and could affect any file at 
any time--I can't back up every file every minute without seriously 
slowing down my/the computer's efficiency and compromising my 
already-tight deadline.

Are there any solutions to this problem? Something I missed or 
misunderstood in the archives?

Thanks for any insight,
Karen