There were errors in the mirror server Windows log, and I assume that it is a problem with the disk subsystem.

Event ID: 129

Reset to device, \Device\RaidPort3, was issued.

Event ID: 129

Reset to device, \Device\RaidPort2, was issued.

Unfortunately, I don't think we resolved it, as issues happen occasionally. But it's beyond my expertise or permission as a DBA. Thankfully, it doesn't happen all the time.