Hello Dale,

this also happens to me sometimes and the culprit was an open process 
still accessing the hard drive. Maybe you can solve it like this:

    $ lsof /mnt/8tb
    zsh       8390 root  cwd    DIR  253,2      4096 27787265 /mnt/8tb
    $ kill 8390
    $ lsof /mnt/8tb

After that, you should be able to close the drive via "cryptsetup".

-Ramon

On 14/06/2021 06:50, Dale wrote:
> root@fireball / # cryptsetup close 8tb
> Device 8tb is still in use.
> root@fireball / # mount | grep 8tb
> root@fireball / #

-- 
GPG public key: 5983 98DA 5F4D A464 38FD CF87 155B E264 13E6 99BF