I have a USB drive that simply will not load on one of my PC's.
The PC simply wouldn't recognise the drive no matter what I did.
So Mr. Google to the rescue.
I found the answer here.
The problem was "More than likely the cause is that Windows renamed the drive to a letter that is already in use. This will happen if you have several card readers, thumb drives or external hard drives attached. It will also happen if you are on a network and have mapped drives".
After following the advice, all was well.