Have you checked if the mount point was successfully created in the jail using the shell?
You should be able to launch a jail shell from the web UI and navigate to the directory where you mounted the dataset or folder for Transmission.
That should be the first step to check if the mount was successful and you can see the files and folders from the dataset or folder you mounted. From there you can check the user and group of the files to see if they are correct.
In my example it looks like in the screenshots below. As said in my earlier comment I still need to figure out how to change the group Transmission uses to write away new files, it's currently set to transmission:transmission.
As a temporary workaround I also created the transmission group in FreeNAS itself with the same GID and added myself in that group also so I'm able to do stuff with newly added Torrents via SMB share.
Even though I'm using 11.2-RC1 the proces is pretty much the same with 11.1-U4 (I used 11.1-U6 on my old server).
- First screenshot are the permissions of the transmission dataset I created
- Second screenshot is the mount point I created with a listing of the files and folder of the mount point
- Third screenshot a view of the transmission user (921 UID) in FreeNAS and the group (921 GID). Inside the jail the transmission user and group have the same UID and GID.