Although the suggestion of using an external thunderbolt connected SSD is a good idea, I would like to point out that on the iMac, and possibly all thunderbolt connectors are a poor design. The cable/connector interface is too fragile and loose in my opinion. My external thunderbolt connected drives are constantly disconnecting whenever I tilt the display or move cables near the drive ends. And yes, I have tried multiple thunderbolt cables. So if you want a reliable boot drive, go with an internal SSD.