I have the iMac 4K. I am booting from an external SSD (cheaper than the apple drives). So, no problem booting into Windows 10 which is on the internal drive (Bootcamp via system preferences). I would like to use 'restart into OS X from windows 10' but at the moment it takes me my internal drive and from there I have restart from system preferences to get back to the external SSD. Is it possible to change this? Or would it just be easier to power off Windows 10 then select boot drive via option at start up?