Can't scan new disk

[Mark Allums <mark@allums.email>]

I just bought a new backup disk, and I want to check it. It's mounted in 
a USB dock.

Running the following gives an error:

root@martha:~# umount /dev/sdb1
root@martha:~# e2fsck -c -c -C 0 -f -F -k -p /dev/sdb1
/dev/sdb1 is in use.
e2fsck: Cannot continue, aborting.

What's causing this and how do I fix it?  It's not MATE; I tried 
rebooting to rescue mode, but that didn't help.

Mark