I would use the m.2 for the OS and some of it form games and programs, then use the old drive for storage, games and apps. You can just mix it up. I had the same board up until recently, I popped in the m.2 drive, booted the machine up and then used macrium reflect to clone my ssd windows partition to the new m.2. Once that finished, i rebooted, went into bios and set the boot drive to the m.2, saved the bios and restarted. After testing everything out while running windows on the new drive i formatted the old drive and used it as storage
