Drives will be formatted when added to a pool. By default, when you use a single disk to create a storage pool and then space, it is locked into Simple (no resiliency). You can later add a second disk ...
My Win 10 system has a pair of 6TB drives set up as a mirrored volume. The downside of this is that when I run out of space I need to replace the pair with an even larger pair of drives. (I could just ...