Some RAID cards cache independently of the operating system's control. You attempt to overwrite the file in place but wind up merely overwriting to its cache in place, never actually hitting the platter on disk where it's stored--at least until the last write is committed. Again, zeros are sufficient, so in this scenario just writing zeros and waiting for the RAID to flush works--sometimes you can't know or force the RAID to flush however (some have battery-backed cache and can even wait to flush *after* a system power cycle w/o losing your bits--thus powering down doesn't always force a flush even).
Furthermore, you have the hot spare situation to consider. The RAID card may have failed a device and replaced it online with a hot spare. You will not generally have access to the failed device, even though it still contains your secret bits. /* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
