One of our servers has been running with a 3-way mirrored Raid-1 volume for a very long time. Occasionally I stop (freeze) one of the volumes and this is later “swapped” with one of the 2 active ones, so that we have a point-in-time backup of the boot-disk. It’s also backed up with Time Machine.
The reason for this is that it is almost impossible to restore certain files – and you will need something to boot from to restore anyway, meaning that the boot disk that’s out of rotation should in theory alway be a “known good” boot disk.
A couple of months ago I swapped one of the active disks to a SSD – figuring that could speed up the server.
And this worked well until Thursday – when the server crashed.
When I tried to reboot from the active mirror this turned out to not work, so I booted from the “known good” disk. And it turned out the SSD had failed. Unfortunately it had failed in such a way that it had corrupted the other disk in the mirror as well.
I tried Disk Utils and Disktools Pro – both told me that the disk was corrupted beyond repair.
It was while I was scratching my head I remembered DiskWarrior – my faithful companion in pre-OsX days. I downloaded (yes I actually purchased a copy), and started it up.
30 minutes later it showed me a repaired directory from the disk, and asked me for permission to write it to the volume. I gave permission, and a few minutes later I had a readable directory from the disk.
I rebooted from this disk (after having cloned it to another volume) – and lo and behold I could boot from it!.
WOW! Thanks DiskWarrior!!!