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Recently I have encountered an issue with steam. If you turn off your computer too fast, steam will register a game you were running as uninstalled. In my case it is GTA V. I have looked far and wide but only found that I should start installing and then steam will recognise the previous files. The problem is, I don't have enough space to start the download to be able to register it. I have slow internet and don't want to wait 5-7 hours to reinstall the game.