Method and apparatus for operating an array of storage devices

A storage controller operates an array of parity protected data storage units as a RAID level 5. One of the storage units is a dedicated write assist unit. The assist unit is a temporary storage area for data to be written to the other units. When the array controller receives data from a host, it first writes the data to the assist unit. Because the assist unit is not parity protected and is only temporary storage, it is possible to write data to the assist unit sequentially, without first reading the data, greatly reducing response time. The array controller signals the CPU that the data has been written to storage as soon as it has been written to the assist unit. Parity in the array is updated asynchronously. In the event of system or storage unit failure, data can be recovered using the remaining storage units and/or the assist unit. The write assist unit also doubles as a spare unit. Data recovered from a failed unit can be stored on the write assist, which then ceases to function as a write assist unit and assumes the function of the failed storage unit.