I don't know if they're using a standard ext3 fs or not; perhaps it is adultrated in some way for their needs that makes it incompatible w/ theupstream tools.You could go through the code to find where that message is printed, then work backwards to why (either via gdb, or printf insertions, or whatever you're comfortable with...)
Thanks, Eric, that's exactly what I've done. :-) Unfortunately there are many different error conditions that could result in an "invalid superblock" message and it seems like it would be a hard job (at least for me!) to work out which one it was as I don't know how to compile a package or even how to get the right source code for Puppy Linux (which I think is almost Debian compatible). I guess this is going off-topic now and I should ask on other lists for help with that? Any assistance appreciated.
David