Jim might be close to it. What are the file names created when you set your scratch disk? Isn't one of them Render and one Waveform?
Could be that the permissions for those drives and or folders got messed up. Would be the answer if client can save to another project... ie 'nother set of folders even maybe on another drive.
But I did find this from v1 days:
http://docs.info.apple.com/article.html?artnum=60277
and this from v4 days:
Audio and video render cache files are now managed separately. Modifying video edits and making changes to video effects that overlap audio effects no longer affects rendered audio render cache files, and vice versa.
and this again from v1 days:
Scratch disk options include:
Capture Audio and Video to Separate Files: when on, video and audio are saved separately, which allows for better performance during playback and capture. To save audio and video files separately, you must specify a different disk for each (no tincluding the system disk).
Video, Audio, Render storage: the disk or folder where video, audio, and rendered files are stored; specify more than one disk for increased storage space. The program automatically uses the specified disk with the most space available (rendering) or the next disk listed (capturing) when the current disk runs out of space.
Waveform Cache: the folder or disk for waveform cache files (graphical representations of audio signals)
Thumbnail Cache: the folder or disk for thumbnail cache files
Minimum Allowable Free Space on Scratch Disks: when disk space falls below this minimum, the program will use another specified disk
So it seems CLEAR to me that FCP manual writers don't know a cache from a file... though in MILITARY terms, a cache IS a storage area... not temporary as it were.
Check the permissions and trash the junk!
Hmmm.
Don't worry Brian... My wife calls me stupid... I think we can take it from Jim.
CaptM