Looks like I underestimated the scale of the problem.
Copying my pictures folder over onto my external drive.
Nothing wrong with that - makes perfect sense. All I do then is point Lightroom to the new location. Oh yes and try to make another copy of my pictures onto my smaller hard drive - this is the thing that didn't work earlier but I might just wipe it clean and start again.
And that is it. Job done.
One small problem.
7 hours 23 minutes remaining......