Here's how to create and mount an encrypted unassigned device in UnRAID.
1) Make sure the unassigned devices plugin is installed in UnRAID. :-)
2) Wipe the device
I do this to remove any old partition tables, etc., and I think the easiest way to do it is on the command line. Change the "of" device to your unassigned device. Confirm you have the right device!
dd if=/dev/null of=/dev/sdb1
3) Create partition table in UnRAID
You can also do this via the command line, but I prefer to use the UnRAID GUI for this to ensure it's created the proper UnRAID way. It will also format it.
4) Encrypt and format the device again
Encrypt it using the same passphrase you used for any encrypted array. Otherwise, it won't mount in the GUI.
5) Try mounting it in the GUI
Try mounting it in unassigned devices. Hopefully it will mount and you're good to go!