There are several causes to failure.
See this post: http://www.internettablettalk.com/fo...&postcount=304
My advice:
Before applying a SSU make a flashable full backup of the system (see http://www.internettablettalk.com/fo...8&postcount=31) and save it on your PC.
Better advice:
Run the system from mmc, you can make a full backup trivially with tar, and restore it by simply unpacking the tar.gz file. You do not need to install bootmenu to boot from mmc, just configure the tablet with "flasher-3 --set-root-device mmc"