Apple delivers software for buidling mirrored software RAIDs with every copy of Mac OS X.
RAID Volumes continue to work even if one drive failes. However, the failed drive should be replaced ASAP. Without RAID Alert, those failures might go unnoticed!
RAID Alert informs you about any RAID systems' status changes (esp. failures) via email, a notification dialog and/or alert sound.
No need to be logged in. RAID Alert's Daemon fully works with no user logged in or on headless systems (without a display).
Try this feature-complete test version!