This is just a public service announcement because a typical N900 user isn't likely to have read enough threads on t.m.o to be aware of the solution to the problem i recently encountered.

Symptoms: The microSD card is suddenly not visible to the N900; the camera app doesn't open when slider is opened.

Cause: The tiny magnet glued to the case underneath the kickstand (it's a ~0.5 cm long black rectangle) came loose and fell out. This magnet is how the N900 senses that the battery cover is on the device...

Fix: find and re-glue magnet or replace with small piece of cuttable rubber magnet material (e.g. printed refrigerator magnets used for promotions).