Comment by PunchyHamster

I had game partition mounted as subpath on a drive and it just not worked well with some apps.

Some apps (in this case Steam) don't run "what is is space in current path" (despise say GetDiskFreeSpaceExW accepting full path just fine), they cut it to the drive letter, which causes them to display space of the root drive, not the actual directory that they are using and in my case was mounted as different partition