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Can't scan new disk

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.