Remove a drive from a volume

How can I safely remove a drive from a volume? I have read everything and searched everywhere and cannot find an answer. It seems impossible to me that it cannot be done as I have a QNAP that can do it very easily. Just click on teh drive and ask to remove from pool, it say ok and out it comes after rebalancing/syncing data. I have tons of space on the 3 other 2 TB drives in the the RAID 5 array.