I apparently spoke too soon as I am clearly not out of the woods yet. Yes, I can now access my plug ins & external editors from LR6, but I just discovered after working on a LR file in a NIK plug in that when I attempt to save the changes, it stops mid way & advises that changes can't be saved as the files are write protected.
I've checked & the external hard drive that I moved them onto is clearly stated to be with read & write privileges, so I am assuming this must be something that LR6 is doing on its own to the files upon my exercise of the "edit in" function of the Library tool bar. But at this point I am stymied. Any thoughts out there?
Lighroom has no control over the read-write status of your files.
If the folder or file has the proper permissions, it still could be that the file is marked "Read Only" in it's properties, and you need to check that and change it if necessary.