Recently I manually migrated from 3 drives to two. One is full and the other has 4TB free, 8TB of data needs rewriting. Copying to the pool rather than in place would also increase file fragmentation and file placement fragmentation, ATM everything is mostly grouped in folders because that's how I organised it before using drivepool, I'd like to maintain that as much as possible but it's mainly just a bit more speed and less fragmentation so I might get away without running a defrag pass.