RAID disk arrays are redundant arrays of independent or inexpensive disks (RAID) that share one or more RAID controllers and function as a single, large and fast disk drive. They can be interfaced to bus-based or external RAID controllers, computer boards and standalone modules that use special protocols to control RAID systems. RAID disk arrays differ in terms of number of disks, maximum capacity, maximum data transfer rate, and operating temperature. Some RAID disk arrays are hot-swappable or include indicators such as lights, buzzers, or email notification. Others include redundant fans for system cooling or redundant temperature sensors that measure and/or control the temperature of the array. In many systems, light emitting diodes (LED) are used to show hard disk read/write status. RAID disk arrays can be attached to a chassis or mounted on a panel or rack. Integrally mounted products are soldered, hard-wired, or otherwise permanently attached to the computer hardware, usually a chassis, which houses the RAID system. Free standing devices are commonly available.