Argh! One of those maddening adventures down the digital rabbit hole later, and the solution turned out to be quite simple. Every time I clicked to import files into Lightroom 5.7 (because yes I’m that old) the program crashed without so much as a smell you later. After checking plug-ins, corrupted preferences files and databases, and avoiding various people suggesting I needed to re-install Windows, the solution turned out to be rather straightforward:
That was all there was to it. Сharging an old tablet was enough to cause Lightroom to fall over itself when presumably being refused access to read the attached storage device.