Cannot access music storage folders on ROCK server from iMac

I recently set up a NUCi7 (16GB RAM and 250GB SSD) for one of my clients. His music is on a USB-connected 5TB hard drive (approx. 15,000 tracks). We wanted to copy the collection to a backup 5TB drive connected via USB to his iMac. After connecting to the ROCK (running the version of Roon OS that rolled out around 7/11/17) using SMB we started the transfer and left to go about our business. He returned to find the transfer had stopped after copying 5 albums. Now when we attempt to connect there is a red circle with a white dash next to the music folder in Finder and we get the message, “The folder cannot be opened because you do not have permission to see its contents.” Remounting the ROCK, rebooting both ROCK and iMac doesn’t change the outcome.