I opened a terminal session through the GUI, and started MC to move some files off of an Unassigned Disc.  I came back a while later to check the progress and the terminal session window was closed.  But, when I look at my discs there are lots of writes going on so it appears that the session is still running.  Is there any way to resume that session?

I'm not aware of a way.

 

In the future I recommend invoking tmux first, that way you have a session manager you can get back to from any terminal. I can't remember if tmux got into the base Unraid, if not you will need the nerdtools plugin. Only install the bare minimum from that plugin, if you don't actively use a command, leave it uninstalled.