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 should simply see the new free space from the new drive.
Since this is a new pool, it's empty.