291966-B21, HP Smart Array 641 Controller. The Smart Array 641 Controller (SA-641) is a 64-bit, 133-MHz PCI-X, single channel, SCSI array controller for entry-level hardware-based fault tolerance. Utilizing the internal SCSI channel of the SA-641 allows you to configure up to 6 internal hard drives to store up to 1.80TB of storage. The 64MB and 128MB (351580-B21) BBWC (Battery Backed Write Cache) Enabler upgrade allows the SA-641 controller an option to add transportable BBWC for improved controller performance and increases the total controller memory, up to 192MB. The SA-641 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.