If you disable write cache for a particular unit (such as a RAID 5 array or single disk), the controller firmware will disable write cache on the controller and on the disk drive.
If you want to disable the cache on the controller, but leave the cache on the disk drives in the unit enabled, then you can use the CLI tw_cli and these hidden commands on 9000 series controllers:
To check the “Disable Force Drive Cache On” policy, use command [tw_cli /cx show forcedrivecacheon].
To set the “Disable Force Drive Cache On” policy, use command [tw_cli /cx set forcedrivecacheon=on|off].
http://www.3ware.com/KB/article.aspx?id=15555&cNode=5T3M5P