I'm having this same problem, Ubuntu, Steam, XP11, Orbx Central 4.1.42, TE, large numbers of fails. Mine are on the same SSD though. Potatophone pic of the error in Xplane 11, which causes it to barf- note it's Oregon HD v1.1.0.
I created ~/.xplane/Orbx/xp11/ for the library install location, everything is there except "Orbx libraries", which is in the xplane dir (that's the only install option).
I do notice the "backup dir" isn't calculated, sizewise:
```
$ du -hs /tmp/Orbx/ ~/.config/Orbx/
72K /tmp/Orbx/
249M /home/USER/.config/Orbx/
```
The size was missing both before and after I did a "clear" and a "verify"; the size of 249mb is after the most recent clear+verify.
Here's what `find` finds for those files:
```
$ find ~/.xplane/ ~/.steam/debian-installation/steamapps/common/X-Plane\ 11/ -name b8d0x4f80x17*
~/.xplane/Orbx/xp11/TrueEarth US Oregon HD/Custom Scenery/Orbx_C_US_Oregon_TE_Orthos/textures/b8d0x4f80x17.JPG
~/.xplane/Orbx/xp11/TrueEarth US Oregon HD/Custom Scenery/Orbx_C_US_Oregon_TE_Orthos/terrain/b8d0x4f80x17SM.ter
~/.xplane/Orbx/xp11/TrueEarth US Oregon HD/Custom Scenery/Orbx_C_US_Oregon_TE_Orthos/terrain/b8d0x4f80x17.ter
```
It looks like the symlink to that dir is correct from the steam install:
```
$ ls -l ~/.steam/debian-installation/steamapps/common/X-Plane\ 11/Custom\ Scenery/Orbx_C_US_Oregon_TE_Orthos
lrwxrwxrwx 1 USER USER 92 Jan 1 12:24 '/home/USER/.steam/debian-installation/steamapps/common/X-Plane 11/Custom Scenery/Orbx_C_US_Oregon_TE_Orthos' -> '/home/USER/.xplane/Orbx/xp11/TrueEarth US Oregon HD/Custom Scenery/Orbx_C_US_Oregon_TE_Orthos'
```
There are about 6300 files in that .xplane/Orbx/xp11/.....Oregon..../ dir. 202 are dds files. There is no "terrain" dir in there.
Next, I cleared the two directories using the Downloader buttons and unchecked both of them ("check disk space", "make backups"). Running "verify" on Oregon still says ~20k files failed to convert.
skimming the massive central.log file (1.5mm lines), I see this occurring frequently:
2022-01-01T18:24:40.774Z [INFO] [Saturn] - System.DllNotFoundException: Unable to load shared library 'libdl.so' or one of its dependencies. In order to help diagnose loading problems, consider setting the LD_DEBUG environment variable: liblibdl.so: cannot open shared object file: No such file or directory
libc6 is installed per the setup instructions, maybe it's this linker problem? (second similar discussion)
```
$ whereis libdl.so
libdl: /usr/lib/x86_64-linux-gnu/libdl.a
$ find /lib /usr -name libdl.so*
/usr/lib/x86_64-linux-gnu/libdl.so.2
/usr/lib/i386-linux-gnu/libdl.so.2
```
That's as far as I've gotten. Out of 122k lines matching "Exception" in the log, after I exclude liblibdl.so, there are about 300 remaining. They are related to download retries, it appears from the verbose stacktrace.