How it works
What Steam opens
C:\Steam\steamapps\common\SomeGame\Unchanged. This is the path Steam 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.
A game installed to your main library lives in a folder inside steamapps\common. Steam stores that path and uses it for launching, updating and verifying. Everything Steam does with the game goes through that one location.
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\Steam opens the path it recorded. Windows follows the junction and returns files from the other drive. Steam 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 Steam 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. 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 the Paid tier 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 the home band to bring that one game back to the Steam 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.
Nothing moves while Steam is downloading, updating or validating. GameHydra waits until Steam is idle.
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. It runs automatically before uninstalling GameHydra, so removing the app leaves no trace and no dependency.
What a junction is not
The free tier manages Steam across one extra drive, with the same mechanism and the same safety rules.