I run into the same issue with an external drive that has a btrfs file system (and had the same problem when the drive was formatted as NTFS and I mounted it as the new ntfs3 Linux file system type).
I work around it by running rsync -avP ~/.var/app/com.valvesoftware.Steam/data/Steam/steamapps/compatdata/$WORKINGAPP_ID/pfx/dosdevices/ /run/media/$USER/$DRIVE/SteamLibrary/steamapps/compatdata/$BROKENAPP_ID/pfx/dosdevices/. After I have done that, the game's installer (vcredist & Co) runs. Everything apart from dosdevices appears to be created correctly.
Steam is installed as a Flatpak here, the external drive is made accessible via flatpak override --filesystem=/run/media/$USER/$DRIVE/ com.valvesoftware.Steam.
I switched from NTFS to Btrfs in the meantime. That did not change anything about the behaviour mentioned above. (cf. https://github.com/ValveSoftware/Proton/issues/150#issuecomment-1024746234) I then switched to Proton 7.0-1, which appears to have resolved the problem.
After installing and trying to run R.E.P.O., I got an error that the dosdevices folder does not exist within the relevant prefix, it was not created at all.
Recreating the prefix did nothing.
The prefix IS on btrfs, but I'm doubtful that the file system is the cause of the issue - I've installed many games on this file system that use proton with no issues.
I used Proton Experimental and the version of steam I'm running is the Arch Linux steam package.
Copying a valid dosdevices folder from another prefix as mentioned above, but this is a temporary workaround as best.
I am not sure why Steam is trying to use proton prefix on the same drive as the game, rather than the using the main drive. Running the game with PRESSURE_VESSEL_SHELL=instead and then pointing STEAM_COMPAT_DATA_PATH to the prefix on main drive will allow the game to launch properly. Seems like a simple fix to work around the NTFS issue.
proton experimentalx1 2025-03proton 7.0-1x1 2022-02STEAM_COMPAT_DATA_PATHx1 2026-06
I have a separate ext4 drive that's exclusively for my games, but when I try to install games with Proton to it none of them launch, becuase
pfx/dosdevicesis empty. Any combination of game and proton version has so far failed to start without manually populatingpfx/dosdeviceswith the expected symlinks. I have two library folders, one in the default.steam-location and the other on the second drive. Installing games to.steamworks fine.All games give error logs similar to this: https://gist.github.com/Kimplul/e8d7fb4dcb2119a9cf6e3893258b812c
And here's my system information: https://gist.github.com/Kimplul/9a5ccd79801550c45d1d2f8cbc0e38da