Did you get this sorted?
Seems to me you did everything correctly.
So, to be clear - You had a standalone 8TB drive that was getting full.
You bought a new 12TB drive.
You downloaded and installed DrivePool.
You created a brand new Pool consisting of your old 8TB drive and the new drive 12TB drive, giving you a new Virtual Drive, G:
Because G: is considered a new drive, you are going to want to MOVE all of your files from E: to G:
That's all you should have to do.
In the future when you add drives to the pool you won't have to do anything and you shoul