I have expanded a RAID 5 to include 12 - 3 TB drives. I was expecting seeing a total of 33 TB in the RAID but only see 30 TB. I do not have a hot spare. Could 2 drives be reserved for parity? This server is a Dell R510 with a H700 controller. It function is my media server for Backupexec. The OS is on a separate RAID 1. The data RAID started with 7 - 3 TB drives. Over the past 2 months I have been expanding the RAID one drive at a time to a total of 12 drives.
raid 5 in this config should only be seeing 30TB usable space because 1/8 of the space must be used for parity.
you can find a raid calculator also by web search.