So, #HackerNews went down, twice, in the period of 24 hours, when first the main server HD failed, then a few hours later the failover server's hard drive also failed.
This appears to have been due to a SanDisk hard drive issue which causes drives to fail at precisely 40,000 hours of uptime.
The issue had been submitted multiple times to HN, though those submissions never made it out of the queue.