My setup is currently 4 drives. All four are WD mybook where 3 are shrugged and used as internal drives but the 4th is not shrugged and is used via USB3. I didn’t have enough sata ports to have all four internal.

Now it’s getting annoying (after 4 years) to have that one external drive. Can I shrug it and use it internally without having to format it?

I use mergerfs and snapraid where one internal drive is parity and the internal and one external are data drives for snapraid. Mergerfs uses the two internal and the one external data drive for one big drive.