Actually - I think I sorted it on my own. I wasn't using the "./move.sh" script with the proper command.
Went and performed the following:
./move.sh &
with the move script containing the following command:
mv * /mnt/HD/HD_b2/media/downloads/
So far it's done quite well moving between the drives, but then I realized I won't be able to easily control it - Local Backups looks to do the same thing, so I'm all on board with that, and I've switched over (after struggling to kill the bloody script I was running!)
Thankfully I've got over 2TB free on the drive I'm moving to, so I'll worry about the extraneous stuff later!
Thanks so much for the Local Backups suggestion, ivan! That's really helped a ton, should have looked there first! This is SO bloody fast in comparison to the windows copy, and looks equivalent to my mv command.
Did I mention you rock? Because you rock.