have you been connecting your phone to a pc recently?
if that so, that might be the cause, and i still have no solution till now except rebooting. any idea would be helpful

(i for once, can successfully disconnect my n900 properly from mass storage mode using either mad developer or pc connectivity, but failed to remember how did i do it )

or if someone can show how to mount sd card (or internal memory) manually in file manager, that'd be great too