So, it's that time of year when I need to make a backup disk of some files on a Windows machine, and I want to use UDF so that I can add to it later. It's refusing to let me create any CDs or DVDs since it says the disk is empty and needs formatting, and then won't let me format them... And yes, the disks have never been used (yet) and are un-scratched and so on. It also won't let me edit some disks that I know were made with Windows' own utility for this (Live Filesystem, which is basically UDF as well), so I'm just going to squint at that and move on. (Interestingly, I can open & modify those in Linux...)
So, I went over to my Linux (Ubuntu 18.04) machine and tried to make a UDF-formatted disk with mkudffs (since multisession disks work correctly with absolutely NOTHING, apparently, at least not anymore [I made one once around the Ubuntu 14.04 days, but they haven't actually been editable since, and it's expensive to test]) and it's superior anyway.
I ran the command
mkudffs --media-type=dvd /dev/sr0
and it dumps an error message "Device is read-only" (I don't have the exact wording, but that was all it said). Running the command as root didn't help, and I know for a fact that this has worked before.
Interestingly, the "copy files to disk and burn" thing built into Nautilus worked just fine. Of course, checking the "Leave disk open for future additions" option didn't actually do anything (no surprise there), but it DID burn the files I had on hand.
Now, for next time, what exactly is wrong that it won't let me UDF-format my darn CDs or DVDs?!
[I have not tested with one of my DVD-RWs, but I have tried both CD and DVD just now (with the appropriate media-type flags) and got the same error. They are a bit old, but they are apparently still writable, it's just that for some reason mkudffs in particular is having issues with them.]