MountFilesystem Failed
If you get this engine error after startup, you either mounted a game you don't own or a game that isn't fully installed/ updated.
To fix this, check the number you get after "MountFilesystem" on this site to see what game the appid belongs to, e.g. "MountFilesystem( 242 ) failed" would be "Counter-Strike: Source Shared Content" and thus belongs to the game Counter-Strike: Source.
If you own the game, make sure its fully installed and updated, easiest way would be to just launch the game once. If you still get the error, right-click on the game in Steam, go to Properties, Local Files and "Verify integrity of game cache".
In case you mounted a game you do not own, go to your "steamapps\SourceMods\obsidian\mounts" folder and delete the files of the game(s) you don't own.
Remember to never mount games you do not own!