I just installed the Transmission-plugin on my OMV.
I created a 'Downloads' folder for the downloads (obviously). Transmission needs read/write access for the group 'debian-transmission' for that folder.
As far as I know I have addread/write access using ACL in 'Shared Folders'. In my case ACL is greyed out so I can't click it.
Is there a setting where I have to activate ACL?