291967-B21, HP Smart Array 642 Controller. The Smart Array 642 Controller (SA-642) is a 64-bit, 133-MHz PCI-X, dual channel, SCSI array controller for entry-level hardware-based fault tolerance. Utilizing both SCSI channels of the SA-642 allows you to configure up to 6 internal and 14 external hard drives to store up to 6 TB of storage. The 64MB and 128MB BBWC (Battery Backed Write Cache) Enabler upgrade allows the SA-642 controller an option to add transportable BBWC for improved controller performance and increases the total controller memory, up to 192MB. The SA-642 also features complete data compatibility with previous generation's Smart Array controllers to ease data migration from server to server and for easy controller upgradeability.