The submountd works not correct as expected. 

Mounted devices/partitions should be umounted after some seconds if nobody use 
the partition. This works only with devices with only one partition. On devices 
with more partions this works only for the last mounted partiton.

Because of this you have in /proc/mounts multiple mounted partitions and you 
must multiple umount the device. For this you must also know the mount point, 
because if e.g. the USB-Harddisk is removed you can't umount with the devpath.