So, a friend has asked me for advice with his 27" iMac. He bought it with a fusion drive, but is looking to go full SSD due to the relatively slow performance. I have explained that the iMac cannot have the drive replaced without some serious surgery on the system and he isn't keen to go that route as it is still in warranty for another 2 years. I understand it should be possible to boot from a SSD connected via Thunderbolt or USB 3. I've got a few questions about this. 1. Will only specific drives do this or can I buy any enclosure or external drive and it will work? 2. Which is better for this - Thunderbolt or USB 3? 3. What sort of performance improvement would be achieved over the stock 1TB Fusion Drive with a current generation SSD (connected via Thunderbolt or USB 3)? 4. Has anyone done this long term? Any pros or cons? Thanks in advance! EDIT: See my later post for steps taken to upgrade to external SSD.