With this optional feature, the IBM TotalStorage DS4500 is capable of supporting concurrent attachment of DS4000 EXP700 and DS4000 EXP100 Storage Expansion Units. Specialized applications may benefit from a mixed drive system, for example, high-access data on Fiber Channel and near-line or low IOPS data on SATA. Sites with budget restrictions may require a mixed system to save the cost of a second controller module. SATA could be used for internal high-speed backups via Volume Copy or backup software.