The Promise FastTrak S150 SX4 is a drive controller that allows one to combine multiple physical disk drives together to create a redundant array of inexpensive disks, or RAID. Typical non-RAID controllers only allow you to use each physical disk drive as an individual drive. With a controller that supports RAID, the drives can be used together to increase performance or reliability.

Whether one increases performance, reliability, or both depends on how many drives one has and how they are configured. It is possible to use a RAID controller like a standard controller, for example if you only have a single disk drive, but the real benefit comes from having two or more drives. Like 3D graphics, RAID management can be done by specialized software that runs on the main CPU instead of by a dedicated add-on card, but there is a significant performance penalty. For example, Windows Server 2003 can create software-based RAID arrays called Dynamic Disks.