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Let's say I have a game bought on retail/other digital download and installed on my hard drive. I buy the same game (or a minor variant of it, say the GOTY edition) on Steam. It should be possible to use the existing installation to avoid the generally big downloads. This is very useful as it saves both bandwidth and time.

Previously, as far as I understood, Steam downloaded the game files directly to steam\steamapps\common\<game_name>. Since this folder would be created on starting the download, pausing and closing steam, copying the game files to this folder and then validating game files would make Steam absorb as much as possible and just download the missing files.

Currently, Steam downloads the files to steam\steamapps\downloading\<game_id> and then moves them over to the aforementioned folder once the download finishes. So first of all, what the final folder would be is not clear. So what would be the correct way of doing the same thing, that is, make steam use as much of the existing files as possible? Should the files be copied to downloading\*\, or should I figure out the actual folder (say via googling) and copy the files to steamapps\common\*\?

More specifically, I had a retail copy of Arkham City installed and I just bought the GOTY version on Steam. Now I tried both of the above things, in either case, the validating does not seem to do anything. Assuming I have the correct folder name steamapps\common\Batman Arkham City GOTY, according to this, if I copy the files there, Steam seems to just continue with its 17GB download. If I copy it to steamapps\downloading\200260 then the download keeps stopping every few seconds with "Disk Write Error" after modifying 1 or 2 files, and I have to keep hitting resume.

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I think your best bet would be to find someone who has it on Steam (on here) and work with them to check the file structure. Steam may arrange the files differently, which would cause it not to detect your files. –  SaintWacko Dec 26 '12 at 14:28
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1 Answer

up vote 12 down vote accepted

I think the following is the correct way to currently do what I wanted:

  1. Ensure that Steam thinks the game is not installed. Close Steam just to be sure.
  2. Copy the game files to SteamApps\common\<game_name>, where <game_name> is to be found out via googling/equivalent. In my specific instance, this was Batman Arkham City GOTY.
  3. Launch Steam and install the game. The installation will go through a "Discovering existing files for _" stage.
  4. Watch and wait as it downloads only the missing parts.

I still have to download 9.8 GB though (instead of 17) in my specific case.

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are you sure it actually downloading only 9.8GB/17GB instead of 9.8 then uncompressing it to 17GB? When you actually get it installed, did it keep your game saves? how big is the resulting folder that holds the steam install? –  Colin D Jan 8 '13 at 19:44
@ColinD If I started the download from scratch, it wanted to download 17 GB, and I know it has downloaded only 9.8 GB using this method. I also noticed that after the "discovering" stage was over, the common\* folder was reduced to around 7 GB, presumably by removing the files that don't match. The final folder size is 18.6 GB. It didn't keep the saves, probably since the Steam saves are kept somewhat differently than the retail version (I didn't need it to anyway, since I was did a fresh playthrough for the Steam achievements). –  ronno Jan 8 '13 at 19:50
thanks for the update! –  Colin D Jan 8 '13 at 19:53
I'm trying to do this (with Thief), but Steam doesn't "see" the files. Is there an actual "Discovering existing files" stage ? –  Manu Mar 7 at 16:53
@Manu There was, at least when this answer was written. Are you sure you are using the correct directory name (<game_name> in the answer)? –  ronno Mar 7 at 16:58
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