Yes. Both disks are Mac OS Extended (Journaled) and are “full read / write”. Both disks are 3TB of size and have (as i write this) still 2TB free space.
And nothing changed between the time it failed and the time it stopped fail, other than that i restarted FCPX. After i wrote the question i have still not experienced any problems (but i have only deleted a couple of projects in that timeline).