Apps · App updates after a move
Why does Storage Studio keep a backup of some moved apps on my external drive?
Short answer: For apps that replace themselves when they update, Storage Studio keeps a compressed safety copy on the fast drive so it can restore the app if an update damages it. It uses no space on your Mac.
Only for apps that update themselves by replacing the whole app in place — the kind of update that can fail partway when the app lives on another drive. For those, Storage Studio keeps a compressed safety copy on your fast drive when it moves the app, so it can restore the app if an update ever damages it.
The copy lives on the external drive, not your Mac, so your built-in drive still gets the full space back.