Spent more time on investigating this problem and it seems that the problem lies with the
location of the audio files as inferred from the pool.
There seems to be wave/aiff
files, which are otherwise playing fine, indicated as 'located' in a missing folder (maybe
it was there and I erased by mistake, I don't know). Common sense would dictated the
samples simply not to work, but they still do, mercifully....
So, when backing
up the project, Cubase freaks out... (no clean up tracks or emptying folders helps). I am
now spending lots of time just bouncing the 'offending' track and erasing them afterwards
from the pool. The only solution. Not very elegant!