Samine problem - I just bought a Seagate Personal Cloud 3T NAS disk for my Macbook Air 11" (2015) OSX El Capitan 10.11.5.
I followed the Seagate quick install but Finder cannot connect. Finder showed a question mark next to "Personal Cloud." Said "Connect Error."
So I read all the discussion above, and installed the Seagate Dashboard http://www.seagate.com/manuals/software/dashboard/introducing-seagate-dashboard/
The Seagate Dashboard takes you through the Seagate install wizard procedure to set up the Personal Cloud Drive. It's very clear and easy.
The Wizard procedure installed TWO firmware updates on the Seagate drive. TWO reboots. Now it works fine for file storage, backup, Time Machine, everything!
My conclusion is that the problem was out of date firmware in the Seagate drive.