あっはーん?chroot抜けるの忘れてた。
root@raspberrypi:~# umount $PI/run
root@raspberrypi:~# umount $PI/sys
root@raspberrypi:~# umount $PI/proc
root@raspberrypi:~# umount $PI/dev/pts
root@raspberrypi:~# umount $PI/dev
root@raspberrypi:~# umount /dev/sda1
umount: /dev/sda1: umount failed: No such file or directory.