mount: /dev/sdb1 already mounted or /mnt busy

Mounting external USB disk on Debian GNU/Linux 7 system I got message:

# mount /dev/sdb1 /mnt
mount: /dev/sdb1 already mounted or /mnt busy

Check isn't it been used by device-mapper [1] with command [2]

# dmsetup ls
33e4d313252504345-part1 (254:1)
33e4d313252504345 (254:0)

If so, release them by

# dmsetup remove 33e4d313252504345-part1
# dmsetup remove 33e4d313252504345

And try again:
# mount /dev/sdb1 /mnt

[1] https://en.wikipedia.org/wiki/Device_mapper
[2] http://linuxcommand.org/man_pages/dmsetup8.html