I just logged onto my server to see one of my 6 drives has failed with an "IO Error". Disparity v0.41 - which is always running - had interpret this failure as a delete of all 9000 files, and was happily ticking away updating the parity set! I cancelled the update, rebooted the server and the drive is back up. Disparity is now updating the parity set with the few hundred files it had just removed!
There needs to be clear code in the update mechanism to distinguish between files being missing and IO errors. Not quite sure how this would be achieved, but it's important!