Hi. I came back from a photo vacation. Put my pics on an external HD (seagate 2 terrabyte), use mac air, up to date os, and LR5. LR will not allow me to create subfolders on external HD, but i can create subfolders on internal HD. Thanks for your help.
Make sure that you have read and write access on External drive.
You are batting 1000. Problem identified correctly. Problem solved. In fact thus solution cured a third problem before, I mentioned it. I had been unable to read another external HD completely. Appears that the transition to el capitan, messed with external hd permissions. Great job diagnosing problems.