Just checked the old backup of Beastbits. It was completely, and utterly, 100% invalid.
For whatever reason, the last backup had gone completely tits up, to the point where Windows could see the backup drive, it could see there was 24Gb left on it. But could it see the 100Gb of data on there? Of course it couldn’t, orther wise I wouldn’t have started off with “Oh my god.” A scandisk of the drive belched up system errors, and even after they were corrected, Windows could still see nothing.

So, *gulp*. I’m formatting the drive, and going to do one last backup of it. A backup which I will no longer trust with any type of certainty at all.
Instead, I’m going to buy a proper external HDD and some type of backup software.