I think that different versions of sonar use different names for the audio content, or a different way to name them, so x3 can't find audio files named, for exemple, in 8.5. Possible? Someone meet the same problem?
 
 

Filenames reverted to 8.3 somewhere along the line?

I think so...for exemple, the project opened in x3 look for an audio file called "1 track drum", but in the audio folder that same file is called "1TRACKD_" so it can't import it during the search process. I'm not sure the exact version used for the back up...surely 8.5, x1 or x2.
 

So it looks like the problem is that the backup software used the short filenames (DOS/8.3) rather than the long ones (NTFS/FAT32).  I think you're out of luck and will need to rebuild the hard way - at least the first 6 characters of the filename will be the same...

I can open "all" 10 years old Projects without any problem from my backup.
 
First: If you have copied the files from DVD go sure, that the files on your HD are not attribute "read-only".
 
Otherwise, something goes unfortunately wrong with the backup.
The Project-filename do not coresspond to the audio-files and do not go together

Sadly, i think the same way! I hoped someone with the same problem had a better solution...but probably it does not exist! Many and many thanks to you!