be sure to have the usb unplugged when trying to make photos/videos. ^^

if no usb-cable is connected, your filesystem may be corrupted.
just connect you n900 in mass storage mode to your windows pc and perform a filesystem check.
under xp: rightclick on the icon of your n900. click on properties and go to the right tab where you can perform the check. set the check to repair failures on the drive.