Hello there.
This is mostly aimed at the devs for them to correct.
It would seem that there is an issue with the launcher that has been downloaded with the Steam client.
The issue many have been facing when they've just downloaded the game via Steam is that when they open the launhcer they're presented with an install button rather than a play button - both my fiancée and myself included have faced this. It would seem that when the launcher is run it checks for the client version in the wrong installation directory, hence why the button changes to an install button. Clicking the install button launches the setup.exe, which the prompts for an installation directory, which is where many has the issue - they don't have enough disk space to begin the installation. However, here is the thing, Steam has already installed a WORKING version of the game. I know this for a fact because I've tested this before I applied the workaround, because I was able to launch the game from this location: \Steamapps\Common\Zenimax Online\game\client\eso64.exe and even login - which proves that it indeed was a working version that Steam had installed, so why the second installation? This supports that the launcher is INCORRECTLY configured in relation to the Steam version installation.
The workaround I applied on my own game installation was running the setup.exe, then copying the launcher folder from the resource.zip located in temp folder when the setup.exe is running.
Launcher folder was copied to the Zenimax Online root folder and exectuted from there, which ran the launcher which updated the launcher - NOT the game and only a few megabytes was downloaded to update the launcher. After the launcher was updated the install button was changed to a play button. HOWEVER, this workaround didn't work on my fincées installation. On her installation I tried several things, including copying specific files from my installation to hers while omitting specific files and deleting specific version files from her installation - which didn't work, so instead I ended up with removing her removing all files from withing her installation folder and then copying my ENTIRE installation to her installation folder. When the launcher was executed it verified the files and downloaded some 8gbs and then then the button changed from install to play.
This for many will be way too much to do just to get the game working when they don't have the space to have the game installed twice.
The fix for this seems quite straight forward - make it possible to change the installation directory from within the launcher WITHOUT having to reinstall nor re-download the entire game.
It may be that I've gotten this to work for me, my fiancée and a few others, but having to resort to workarounds just to be able to play the game really doesn't seem viable, and many would probably just uninstall and not look back, unless they really want to play the game.
So devs, fix this ASAP.