Removing healthy drives from a Storage Pool without causing data loss
- Under Physical drives for the storage pool you want, click/tap on the Prepare for removal link for the drive you want to remove. The drive will now be listed as Preparing for removal.
- Leave it until the drive is ready to be removed. This could take several hours, depending on how much data you have stored there.
- When the drive is listed as Ready to remove, click/tap on its Remove link.
P.S. Don't use "simple" VD in Storage Spaces, you have a big chances to lose all your data.