FortiMail models with a hardware RAID controller have a hot spare RAID option. This feature consists of one or more disks that are pre-installed with the other disks in the unit. The hot spare disk is idle until an active hard disk in the RAID fails. Then the RAID immediately puts the hot spare disk into service and starts to rebuild the data from the failed disk onto it. This rebuilding may take up to several hours depending on system load and amount of data stored on the RAID, but the RAID continues without interruption during the process.

The hot spare feature has one or more extra hard disks installed with the RAID. A RAID 10 configuration requires two disks per RAID 1, and has only one hot spare disk. A RAID 50 configuration requires three disks per RAID 5, and can have up to two hot spare disks.