Hi, I am thinking of getting a new 27" iMac. As I understand it, the hard drives inside are not user upgradable anymore. So is it possible to get an external SSD and boot from it and use it as the boot drive? And then use the existing 1TB drive as a storage drive? If so, will the boot up and other speeds be as good or almost as good if it were an internal drive? Assuming of course if it is connected via Thunderbolt or USB 3.0. Thanks!