I normally work with LR on my desktop PC where all my images and my LR catalogs are on a second HD in the box. Work fine no issues.
Just recently purchased a laptop and would like to be able to work with images in the catalog on this new machine. So I copied all my images and catalog to a 500GB external drive. When I try to access the catalog with LR4 I get the message below.
I can write to the external drive so I don't know what the problem is.