The same exact settings have been configured for Radarr (also Docker). Same PUID and PGID too.
Despite all this, Sonarr reports that there’s a write error for the user. I can confirm it’s writing because Google Drive (using Rclone) has the write test file.
I figured it out and want to report here to help anyone else that may find this thread. The issue was with my rclone mount configuration. There seems to be a difference between how Radarr considers the directory appropriate and how Sonarr does. The write check may not be happening on Radarr(?). I don’t know. Either way, adding the below helped with this.
rclone mount -v --allow-other --vfs-cache-mode full --dir-cache-time 48h --vfs-cache-max-age 48h --vfs-read-chunk-size-limit 512M --buffer-size 512M gdrive: ~/mnt/gdrive
Thanks @markus101 for pointing out that the file was not being deleted which lead to look into rclone some more.