the auto-off issue could be caused by the battery which moves inside its housing, i've resolved putting a little piece of cartonboard behind it.
if it's the battery then you shoul hear some noise when you shake the n810.
when you try to connect via usb the system umount both the internal and external sd cards, the message you get means that one of the cards was busy and the process was interrupted.
if you have the debian chroot open you sould close it with closechroot or it won't let you umount the card.