If you are like myself and use a modist sized drive for windows and use a larger drive for gaming you generally want to install your games on your storage drive not your boot drive, but as it stands by default when you goto use the xbox beta app it wont allow you to change from its default installed drive.
A easy to fix this is to goto system- then storage- and under the storage tab find " change where new content is saved" then select "new apps will save to" and select your preferred storage drive.
Then uninstall your xbox (beta) app, and reinstall it, now when you goto install a new game on the pc game pass it will give you the option to select what drive you want to install it on.
This issue happens because by default windows needs to recognize that your pc has the folder called windows app on that drive and that it has been setup by a windows app installer atleast once, for the xbox (beta) app to recognize the new (useable) install location for your games and apps
Hope this helps everyone :)