aufs mounts won't umount due to being busy
I've installed aufs in order to seemlessly union two filesystems over /usr. One branch is the plain old, existing /usr directory; it's not mounted from a separate filesystem or anything. The other is a /usr on a microsd card.
This is what I've added to /etc/fstab:
#the microsd mounted on /aufs/internal_microsd
/dev/mmcblk0p1 /aufs/internal_microsd/ ext2 defaults,noatime 0 0
#the aufs /usr union
none /usr aufs br=/usr:/aufs/internal_microsd/usr,defaults,noatime 0 0
This works--they are mounted correctly--but during shutdown (when the mounts are automatically umounted and then remounted read only). the umount complains that /usr, /aufs/internal_microsd, and / are busy (and it does not umount them). This causes errors on the next bootup: 12 orphaned inodes are deleted, and all other partitions on the disk which the root filesystem exists on are not mounted (other entries in fstab for the same disk).
(I would copy and paste the actual errors, but I don't know where to access a log for them)
Note: I've tried remounting the aufs mounts readonly prior to shutdown (as the aufs author recommends), but the associated commands issue the same "is busy" error and does not remount them.
Why are these filesystems refusing to umount due to being busy, and how would I fix the problem?