Default [Q] problems after omnirom install and fallback

Hi,
I've got a xoom eu, the tablet was flashed for long time with different rom (CM, EOS9), I was using TWRP.
Some days ago I've installed TWRP bigpart and latest Omnirom, everything was working fine.
Before the procedure I've made a nandroid backup of the CM 11 and TWRP that was running before the new installation.
After some days of playing I've decided that I prefer the old CM, after taking a second nandroid backup, containing Omnirom, I've tryed to restore the first backup....
A this point everything is screwed up, no recovery, no boot.
I've restored with RSD the original stock HC rom, this works well, after I reinstalled TWRP bigpart but if I try to restore one of the 2 nandroid backup the process failed.
It seems something got messed up in the partitioning scheme.
So what I can do to restore "original" partitioning scheme? At this point I can continue to use the partitioning modified by the bigpart procedure.
Thanks for the help.
diphe