I' am not a power user, and I'm not a developer but I do know how to reflash N900 if something goes wrong.

So I own 3 Nokia's. One of them is for testing and reflashing, one I use for my job and one of them is for parts but it is flashed with latest image and nothing else is installed on it. I bought 64GB SDXC-UHC San Disk card and when I put it the phone that has all kinds of software installed, nothing happens, not even errors that some of the users report, nothing at all.

So if You have that kind of a problem, and U do own another nokia that has basic image, it will be recognized with that error "unsupported operating system" and U just format it as described and put it in the other Nokia and it will work fine. Why, I do not have a clue, but I suspect it has something to do with kernel I'm using (CSSU-test). Also you could try formatting it with some kind of third-party software.