At some unknown time in the past 6-8 months, Retrospect Express HD (the freebie backup program that came with the external backup drive I bought) just sorta died, without any explanation or notice. It was only because I've been burned by exactly that kind of thing before and check on it periodically that I discovered the problem before having lost all my data.
However, as I'm sure you're aware, I've been kinda busy lately. So I haven't had a chance to fix the problem even though I knew it was there. That makes things doubly dangerous -- Murphy, the God of Bloody-Minded Perversity, being what he is, is most likely to strike during that period between when you've noticed a potential disaster, and when you take steps to prevent it.
However, over the years I've gotten pretty good at being bloody-minded myself, and took steps to thwart Murphy. I put "fix backups" on my Too Much To Do List with a due date of Saturday, February 17. That meant that if the hard drive was going to fail, the most GRRR! time for it to do so would be Friday night at around 11:30 -- so I downloaded a new backup program and installed it today. So far it seems to be working fine, and I have backups again.