I wanted to switch the FreeBSD media server to Void linux so I needed to be sure I could move the media drive over.
I shutdown the Plex server: doas service plexmediaserver stop
and the Syncthing daemon: doas service syncthing stop
I might explore FreeBSD again someday, but I've been using Void on my desktop for a few months now and it seems to be "the best of both worlds" -- BSD and Linux -- and I made the mistake of installing a less supported BSD branch so updates aren't as point-and-click (CLI) as I'd like, and I'd have to compile from source to get back on the correct branch. No thanks.
Once the daemons accessing the "tank" pool were stopped I could run zpool export tank, I should've probably run zfs offline tank as well, but I just unplugged the external USB drive and plugged it into my Void machine.
On the Void machine I installed ZFS sudo xbps-install zfs, remove your old kernels before doing this with vkpurge because it'll try and squish the ZFS module into EVERY old kernel you have.
After a few false starts I created the "/tank" directory and by running:
$ sudo zpool import -d /dev/sda tank
it automatically mounted the dataset to the expected directory and I was able to watch a video with no problems! Easy!