Weird. I stopped spotlight and restarted the machine, then plugged in the disc. All was fine. So I rebooted with the drive plugged in... all was fine.
So I ejected the disc, unplugged it, started spotlight, restarted the machine, plugged the disc back in, all was fine... nothing going mental in activiy monitor. Navigated up, down, all over the place through the drive structure, all was still fine.
So I restarted the machine with the disc plugged in, all was fine.
And all still is--touch wood.
Only changed variables are: there are loads more files on the disc now; and I used a different, shorter, cable to connect the drive up.
I've not encountered cable issues with USB before, except a dodgy ribbon cable in an external USB HDD enclosure, but that looks the most likely explanation.
Which is good, because if this kind of behaviour had been caused by spotlight I'd have been mightily unimpressed :-)
Cheers again Simon.