I am getting there. This morning I managed to use my new external hard drive with all my pictures and my Lightroom Catalogue only on it.
The old external hard drive that I had been using is disconnected.
All I had to do was connect the folders in the catalogue to the actual folders containing the images.
Next I started Lightroom, and quickly managed to import the images that would not roundtrip back from Photoshop into Lightroom before. Thankfully I saved them to my PC hard drive.
And now where am I? Moving all the files from the Pictures folder on my computer hard drive to my back-up hard drive.
There is over 100GB of data in that folder, and I cannot work out why.
If I have done everything else correctly those files can all be deleted. For now they are being moved onto an external hard drive. I am not too good at deleting stuff!
3 1/2 hours to go on that, then I will see where I am up to.