Last night my PC decided to slow down a bunch and then take ages to boot to what seemed like a black screen. Having left it running overnight It did show the desktop this morning. Turned out on of the drives was doing whatever at 100%. After a bit whatever process was doing stuff has stopped or at least the problem had receded for now.

So that's how I ended up backing up shit before breakfast (last one was ~7 months ago.) Not particularly fun to feel like losing my sneepsnop with a day to go.