Firstly, it has happened to other people e.g.
this reports a WADM backup stopping at exactly 9.02GB.
I had a look at the manual and a few other bits, As far as I can tell, this WADM works the way it does as an anti-piracy measure, encrypting the files in to one big lump so no-one can get to them. I don't know why they bother, frankly. That thread after the link notes that someone's written a program for getting to the MP3 files directly, called
WACHandler. I would try that if all else fails - but I can't vouch for it in any way. It won't back up the MCi500H Album Art, for example.