Using FMK and binwalk, I was able to chop the header and footer off of my Swann DVR's firmware and then mod the Cramfs to update the root password. When I reassemble it with FMK, and then run binwalk on it, it looks identical to the original with all sections ending/beginning in the sample places. The contents of the Cramfs do make it a smidge larger, but it still fits into the original "boundaries". When I go to flash the firmware though, it fails after thinking about it for some time. I am guessing that there is a checksum somewhere, or some other security feature. I have no idea how to proceed from here though. Ideas???
Any progress (or luck!) with this?
I would also guess it's a checksum issue.
It does sound like it's probably a checksum / integrity check failure.
Resolution would require further evaluation.
Your best bet is to submit it as an issue at the Google Code site for the FMK.