Default HTC "OS.nb.extra" file structure & contents
I'm examining OS.nb.extra file extracted from HTC HD2 (Leo).
Structure of the file is relatively simple: two int32, fist one is flash block number (continuously increasing, i.e. 1, 2, 3,...), second is "FF FF FB FF". Thus we have 8 bytes per single 2 Kb flash block (of payload).

Last ~10% of the OS.nb.extra file contains just "FF FF FF FF" in both fields. Just as in OS.nb.payload file, which also contains 90% of useful data and 10% of "FF FF FF FF".

Can anyone explain what is there in OS.nb.extra file? I guess it is somehow used to verify flash memory. Is there 'bad block' bit or something else?