I deleted one copy of the backup, as I've done in the past and made a new crontab entry to try the backup again 2 or 3 minutes later. It was still running several hours later, which couldn't possibly be right. Next, I tried clearing the backup on the backup drive and manually copying directories to it. That didn't work too well, either. At some point, I tried to send an email using SeaMonkey and couldn't, because it was unable "to write a temporary copy". I quickly found that that wasn't all it couldn't write, too.
Run df and check the size of /dev.
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