It is often best to have only the OS drive connected during windows installation. There is no way to uninstall windows from the old drive. You can delete all it's file and folders but some are almost always overlooked. Once your OS drive is windows and driver updated 100% you can back up what you want from the old drive and format it to use as whatever type of storage you want. Remember you will need to reinstall your mail client, address books, device drivers and software (printers, keyboard, mouse, cams, etc.) and any programs you had before. If you have pic., music, videos or other media and want to store them (back them up before format) on the HDD, you will need to set windows My Music and similar folders to look in the new locations for media content. None of this is hard just takes a little time.