bad superduper backup - null files

[FSU_Math]

the drive is the one I've used for a long time, never had a problem in the past (not that past performance is any indication of the future). The only mildly negative thing I can think of was that the drive was nearly full after the image was created (the one that turned out bad). Could that have caused it, ie filling the drive but not receiving or misinterpreting the error?

It's my understanding that superduper dynamically requests the space allocation, so could the files in question be at the "end" of the backup and so have fallen off the end? A bit more information, the back up script was a "backup - user files" and the files in question are in the Desktop directory of the last user alphabetically. Does Superduper run through the file list alphabetically or by some other method? I'm trying to find a reason as to why these specific files (and not others on the image) are corrupted.