I had this problem with my Music Library on my Synology NAS server. Eventually the Sonos Tech Team came up with an answer that worked for me. The problem was I only had one user account for the NAS server and all my Sonos Speakers and iMac and PC etc used this account. It appears the NAS drive thought this was a security risk and stopped my two One SLs for accessing the drive. The other speakers worked OK. The solution was to create another user account specifically for Sonos. As soon as this was created and the library rebuilt with the new account all the speakers worked correctly and I can play music from the library from any speaker. Hope this helps with some of these access problems.