Okay, as promised, here is the slightly longer postmortem. Sorry, no timestamps like all the Cool People, too lazy for that.
1) What happened and why?
The primary HDD that contains the postgres database and transaction log ran full because the transaction logs did not get deleted. This was because I have transaction log archiving set up, which allows for point in time recovery of the database, and the archival command (that copies the log files to an off-site location) failed for an