Mirror: Source drive doesn't exist, target drive wiped out!

I'm running 3.0.2 on XP, with this config:
D: drive - internal 500GB
L: drive - internal 750GB
K: drive - external USB 500GB
S: drive - external USB 2TB on another PC, connected via WiFi

My strategy was (simplified here):
1) specific D: directories mirrored to L: (about 50GB)
2) specific L: directories mirrored to K: (about 300GB)
3) specific L: directories mirrored to S: (about 300GB)
Job run via scheduled task every night.

Most of my files don't change on a regular basis (mp3's, big zips, etc), so I'm usually not mirroring the full amount every night.

The incident:
For some reason, the L: got "lost" - there was no longer a drive letter, and the drive itself didn't show up in Disk Management. So #1 failed, but no data loss. However, instead of failing out, steps #2 and #3 wiped out the specific directories on K: and S:. After a reboot, my L: drive returned (with all data intact), and I'm now in the process of backing it up to K: and S: again before I run a big diagnostic on L:. (S: drive temporarily moved to this machine to use USB instead of WiFi).

So my question, - how do I get toucan to kill a mirror operation when the source drive doesn't exist? That should be the whole point of the mirror, right? to be another copy in the event the source is lost.

I don't use equalize for this because I only use K: and S: for backup purposes, and I often delete/move/rename the files on my source drive. Equalize would copy all of those changes back.

Thanks for any help/suggestions/guidance!

- Kaufman