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My friend just bought payday 2 yesterday, and he doesn't want to download it because it will take a long time.
Let's pretend that there are two subject here, A and B. Both have payday, but only B has downloaded it.
A takes the game data from
Steam/SteamApps/common on B's computer and copies it to the same location on his own. He doesn't know about the appmanifest so he doesn't copy it.
I asked him "Did you copy the appmanifest too?". He said no, so I uploaded my appmanifest (for Payday 2) so he can download it. I told him to place it in the steamapps file and he did.
When he restarted steam, the game was still not installed. How can he install it?