I'm using Spotify version (installed with Snap) : 184.108.40.206.g6bd7cc73-35 (6) on Fedora 27 KDE.
I can't store offline songs on an external mounted partition. In spotify, the mount point (/mnt/my-drive) isn't displayed.
It's a problem as I'm using a small SSD for my OS. I'd like to use my second hard drive that is bigger.
This other drive is mounted on /mnt/my-drive with 770 permissions, and the owner and the group is my main user.
I've seen it would be related with using the removable-media connection.