Thanks
That didn't work as I am only changing from one hard drive to another but I did get it work in the end.
I tried moving the library to its new position then I tried opening it in calibre and it gave me the option of looking for it as it was no longer where it expected. so I entered new address but it kept coming back with an error.
So it also gave me the option of forgetting the library which I did then I added it back at the new address.

I was a bit worried that it would forget all the metadata but it seems to have worked OK
I now have a bit more understanding of how Calibre works
Thanks again
.