Online storage capacity expansion of a raid storage system

The methods and structure herein provide for expanding the storage capacity of a RAID storage system while maintaining the same level of RAID storage management. A RAID storage controller may be coupled between a host computer and a RAID storage volume. The RAID storage controller manages the disk drives of the storage volume to present a single logical volume of storage to the host computer. When a storage expansion is desired, the RAID storage controller may communicatively couple to at least one expansion disk drive and begin transfer of data from the original RAID storage volume to the expansion disk drive(s). During this data transfer, read and write operations are continued to the original RAID storage volume. Additionally, the RAID storage controller duplicates write operations to the expansion disk drive(s) such that general storage operations required by the host computer are continued.