Before anyone tries to flag this as a duplicate, know that I have already seen How can I move an installed game from one Steam library folder to another. This question is in relation to the top comment on the top answer to that question.
I have been manually backing up my less played Steam games to a large SAN device. I do so by simply moving the game folder to the share and then uninstalling the game in Steam. I do this because I can afford the space and don't want to wait for the compression on top of the transfer time.
Up until recently I haven't been so short on space that I have run into this problem, but now it's happening more frequently. As the comment in the linked answer states, you need to have space enough to install the game after you have already copied the contents to the library. This is because Steam will initially check the free space before allowing you to initiate the installation, which will eventually discover all the files there anyways.
I understand this check is there for a reason, and it's a perfectly good check, but I am trying to find away around it, or to trick the installation process.
I have tried initiating the download, pausing right away, copying the files over to the install directory, and resuming. This doesn't work because Steam downloads to a temporary folder before copying to the final install.
I have tried doing the same thing, but overwriting the files in the download folders. Steam just continues downloading and overwriting the already completed files because it doesn't check them first.
Is there a way to reinstall a game from a manual backup, without needing twice the space of the files free on a drive to do so?