I'm not 100% sure what your situation is but here is what I do when I replace a hard drive with a larger hard drive.
I have a USB hard drive dock connected to the system. Something that you can stick a hard drive into.
Have a 3-2-1 backup setup and verified
I turn the system off
I remove the old hard drive you want to remove
Place the old hard drive into the USB dock
Place the new hard drive into the spot the old hard drive was in
Turn on the system
Verify that the drivepool pool is still working
Add new drive to drivepool pool
Rebalance the drivepool pool
Click remove in Drivepool for old hard drive.