kpartx: cannot unmount disk after mappings have been deleted

I cannot unmount a partition on which I created and then deleted mappings with kpartx: # mount /dev/sdb1 usb/ # cd usb/ # kpartx -a -p sda -r -s -v sda.img add map loop0sda1 (254:5): 0 16777216 linear 7:0 2048 add map loop0sda2 (254:6): 0 16777216 linear 7:0 16779264 add map loop0sda3 (254:7): 0 440401920 linear 7:0 33556480 add map loop0sda4 (254:8): 0 26159792 linear 7:0 473958400 # kpartx -dv /dev/loop0 del devmap : loop0sda4 del devmap : loop0sda3 del devmap : loop0sda2 del devmap : loop0sda1 # cd .. #umount usb/ target is busy. as far as i can tell, the command kpartx -dv /dev/loop0 properly deleted the mappings, as I can see they have disappeared from /dev/mapper/. But still I cannot unmount the disk, because it reports: target is busy. lsof shows no files open from that disk: # lsof +D Why cannot i unmout the disk after all mappings have been freed up ?