No experience with WIN 8. In Win 7 you needed to repartition your hard drive into two drives (or more). Select which drive is to be your Data drive and create a foldser "My Documents". In Win Explorer go to the original My Docs Folder and right click on it. Select Properties and then the Location Tab. Change the destination to the new My Docs folder and allow the files to be moved. Hope it works in Win 8 as well, can't see any reason why not but stranger things have happened.
Surface Pro (2017) vs Surface Pro 4