Is it safe to use write-back caching on my RAID controller?

A. Probably. In the old days (say, around 1997), Microsoft recommended turning off write-back caching on RAID controllers because when caching fails, it can corrupt Exchange Server databases in a way that makes it almost impossible to recover data. Bugs in some vendors' controllers and chipsets made cache failure a fairly frequent problem, so Microsoft recommended turning it off.

Microsoft’s recommendation is no longer in force. However, if you have a controller that supports write-back caching, you should make sure that you have a battery backup. The backup unit will preserve the contents of the controller's onboard RAM even when the power fails, so you won’t lose data due to a sudden outage. Note that even with a backup battery, you can still lose data—say, if the controller itself fails and you have to pull it—but a backup unit is a good safety measure nonetheless.