When turning on fast-write cache, you have to make any file-systems assigned to those adapters are un-mounted, then make sure that you vary-off and volume groups that are assigned to those adapters (disks). Basically before you can turn fast-write on you have to make sure nothing has a lock on the drives assigned to the adapters. It’s best to turn this on when you first receive your server. Hope this make sense.
Vantandem