How it works
What your launcher opens
C:\Steam\steamapps\common\SomeGame\Unchanged. This is the path the launcher recorded at install time and the only one it uses.
Where the files are
D:\GameHydra\SomeGame\The whole game, on the drive that suits how recently you played it. Never split, never partially moved.
What a junction is not
A game installed to your main library lives in a folder the launcher records, and it uses that path for launching, updating and verifying. Everything it does with the game goes through that one location. The paths below are Steam’s; Epic Games, GOG Galaxy, EA app and Ubisoft Connect record their own the same way, and every step after this is identical for all of them.
C:\Steam\steamapps\common\SomeGame\The whole game folder is copied to an included drive, and only then removed from the original location. A game is never split between drives and never left half-moved — if the copy is interrupted, the original stays exactly where it was.
D:\GameHydra\SomeGame\An NTFS junction is created at the original path, pointing at the new one. A junction is a directory-level link that Windows resolves inside the file system itself, below the level any application sees.
C:\Steam\steamapps\common\SomeGame\→ D:\GameHydra\SomeGame\The launcher opens the path it recorded. Windows follows the junction and returns files from the other drive. It does not know a junction is involved, because nothing in the file API tells it. Launching, updating, verifying and uninstalling all behave normally.
A junction occupies a directory entry and nothing else. The gigabytes are on the other drive. The free space a launcher checks before allowing a download stays high, which is the number that decides whether the next install can start.
Which drive
Kept on the fastest included drive with room for it. Thirty days is the default, and you can change it in Settings. Load times are the thing you actually notice, and they are what the fast storage is for.
Moves down towards your slowest included drive. Still installed and still one click from launching, just not occupying your quickest space. On Standard it can also be compressed in place.
Drag a game onto a drive and it stays there whatever the sorter would otherwise do. Drag it onto your home drive to bring that one game back to its install drive.
Safety
A move is deferred if the game is running. GameHydra checks before it starts and abandons the attempt rather than touching files in use.
A game its launcher is still installing or updating is never moved. Background sorting also waits while Steam is downloading or updating.
The original is removed only after the copy is complete. An interruption — a crash, a power cut, an unplugged drive — leaves the game intact at its original location.
Bring everything home returns every game to its own drive and removes every junction. Uninstalling GameHydra does not run it, and a moved game keeps working without the app, so press it first if you want everything back where it started.
The free plan manages one launcher across one extra drive, with the same mechanism and the same safety rules.