RAID 5E, RAID 5EE and RAID 6E (with the added "E" for "Enhanced") generally refer to different types of RAID 5 or RAID 6 with a hot spare. With these implementations, the hot spare drive is not a physical drive. Rather, it exists in the form of free space on the disks. This increases performance, but it means that a hot spare cannot be shared between different arrays. The scheme was introduced by IBM ServeRAID around 2001.