@netbsd Oh wow, I had no idea there was work done for a fsck_udf! I trialed UDF in 2018 when I wanted a performant shared data partition between Windows and Linux dual-boots (before I ditched Windows entirely). I had to drop it because eventually Windows would stop mounting it as read-write for unknown reasons. Windows chkdsk mostly removed data instead of fixing anything, and fsck from udftools was barely functional (it was not finished by any stretch, so no surprise there). I ended up going with exFAT using the Samsung Linux driver, back when it was in someone's dubious GitHub repo before it got upstreamed.

The state of most UDF implementations really is a shame. It has all the features one would need, and even has provisions for encoding Windows NT ACLs, which Microsoft never bothered to implement... When it was working in my setup, though, it was great! I/O was no issue, files from Windows ended up with reasonable default POSIX modes and nobody/nogroup as one might expect, etc...