[Mac Photos Library in Dropbox Folder - external drive error]

I have my Photos Library for the Mac Photos application in my Dropbox folder located on an external Lacie Thunderbolt drive. Accessing the files from the Photos app works fine as does the sync and other functions. Just purchased Affinity Photo and I cannot open a photo from that location from within the Affinity app. It sees all the folders just fine but the thumbnails for every photo is a grey square and if I click on any name to open it, I get an error message that says "The file cannot be opened because permission has been denied". My user account on the Mac has read/write rights to the location. Opening up a photo on the local hard disk work fine.

 

Is there anything special I need to do in irder to open/edit files in Affinity Photo when the files are located on an external disk? Special permissions?

 

Thanks