Actually, when the N900 is connected to a PC in "mass storage mode", the entire partition containing the photo directory is remounted in such a way that only the PC can write to it. If the N900 got stuck in this mode (maybe by disconnecting it from the PC while it was still busy with a file operation or something), you can remount by connecting it to a PC in mass storage mode again, "ejecting" it, and then disconnecting it. (Also, rebooting the N900 would remount the partitions.)