I am unable to export jpegs or dngs to an external hard drive. I can export to a flash drive and the local drive. I get a message that says "Some export operations were not performed", "This file could not be written".
The hard drive has plenty of free space (3 TB) and the permissions are set set to "Read and Write.". I am on a MacBook Pro running El Capitan and the Lightroom version is 6.21.
May sure the user account you are singed in with, has the read and write permission on the external drive.It seems like that it is a permission issue on the external drive. As we are able to export the images on other locations
Please read my post, it addresses both of your questions