Sarafan: As Caesar mentioned...

Sarafan: As Caesar mentioned the best is to backup installers. This way you don't have to worry about missing dll files as they are usually installed with the games. I don't understand why do you prefer doing a backup of fully installed games.

Caesar.: I'd say the best way is to back up the "offline backup game installers" (formerly known as classic installers) in an external hard drive and/or flash memory, so you can reinstall your games from scratch if necessary. These are the aame files that the GOG Downloader downloads.

There is at least one Python script in the forums that does this automatically (gogrepo), but I haven't used it yet.

Sarafan: As Caesar mentioned the best is to backup installers. This way you don't have to worry about missing dll files as they are usually installed with the games. I don't understand why do you prefer doing a backup of fully installed games.

ariaspi: This is not really a simple question as there are lots of different games built on different engines, each with its own quirks.

Some games (usually older ones) require registry entries, which store paths, serial keys and other things. I might be wrong, but I think some DLCs for newer games sold on GOG are activated only through registry.

Regarding the missing .dlls, these are usually related to redistributables, codecs or CD copy protections (you'll probably need noCD/crack files here).

Here is a list of the things I could think of that anyone should back-up.
- last DirectX redistributable (directx_Jun2010_redist)
- latest versions for Microsoft Visual C++ redistributables (MSVC 2005/ 2008/ 2010/ 2012/ 2013/ 2015/ 2017)
- Microsoft XNA Framework Redistributable 4
- OpenAL
- Nvidia PhysX
- .NET Framework/ dotNetFx 4.5 and maybe 2.0
- Windows Media Video 9 VCM codec (wmv9VCMsetup) for some older games, and maybe Windows Media Format Runtime (wmfdist11)
- newer Unreal Engine games may come with UE4 Prerequisites (UE4PrereqSetup).

ariaspi: You're welcome.

Did you solve the Race Driver: Grid problem? Maybe you missed to install OpenAL.

nightcraw1er.488: Backing up game folders is really a bad idea. Here are some reasons:
- the space taken up by a folder versus the installer tends to be far larger, hence your storage cost will be greater and copying/backing up will be slower. This is exponential the more you have.
- the folder is unlikely to have everything necessary, cpp extensions, ref entries, ini files etc. are all over the show and whilst it may work, you could get untoward crashes and things without the ability to do a clean install.
- modding or patching may become impossible without the ability to do a clean install
- this is more of an aesthetic (and could it be a possibility for fragmentation or but rot?) but one or a couple of files in a folder is far more manageable than many files/folders. One accidental drag drop of a game file from one folder to another and goodbye game.

To conclude there is zero benefit to keeping installed games, only installers. It's really not difficult to download some files, once you are up to date then it is no effort, although catching up is. You can use gogrepo to download all your collection if need be.

Oh, and one other thing, game folders can get clogged up with logs and temporary files, another reason for clean install.

vidsgame: Yes, I did solve it. But no it was some dll file that started with an m like mscvk45 or something. Haha.vidsgame: I find many of these to be fair points and while I do indeed agree that a clean install might do more harm than good but especially with old games, this is potentially difficult especially when you are trying to play a vanilla game and you have to modify one file in the install to fix an issue with said game. Example just off the top of my head would the first Witcher with the audio video issues in the cutscenes and OpenAL as Ariaspi mentioned in the case of GRID. Beyond Good and Evil is one of the worst offenders I've seen with cutscenes but I digress as these are problems with the games themselves rather than your idea but the point I'm trying to make is once you figure out a solution to make these games behave it is often hard to emulate it with a clean install. Especially, when you're scrambling around trying to find a specific fix you applied eons ago same idea could also be applied to modding games.

nightcraw1er.488: This is why I keep install instructions, and such like with the installer. For instance Oblivion, I have probably 5-10 setups with 100-200 mods each, I could keep the folder with each of those setup, or one installer, the mods, and a guide for each setup - a text file with steps. Am just thinking how much it would cost me to keep all mine expanded and modded, I would likely need a server central in the house just for endless hard drives.
Even big world setup for baldurs gate, it's around 32gb installed and modded, around 8gb installer plus mods.