Re: Sims 4 repeatedly installs to both drives
Have a peek under the Origin > Application Settings > Installs and Saves
Make sure those are set to the D drive.
You'll also have a The Sims 4 folder in your Documents, this is normal and you can't really move that one with a setting.
This is a rather old link that worked for someone to move the entire Documents folder, but I'm not sure it still works:
http://modthesims.info/wiki.php?title=Game_Help:Change_Location_of_Downloads_Folder

