I have an Iomega Mac Companion that in a previous life had a 3TB drive in it that I've replaced with a 256GB SSD. The drive has Firewire 800 and USB 2.0 interfaces. It will boot off the USB interface, but for the life of me, I can't get it to boot off of Firewire. I've tried holding down the Option key while booting, and the drive does not appear in Firewire mode, but does appear when attached via USB. When connected via Firewire, the drive is visible in the Startup Disk preferences pane, but still will not boot via Firewire. I have a mid-2007 24" iMac with OS 10.11.5. I've tried resetting the SMC and PRAM, but it still will not boot off Firewire. I'd prefer Firewire 800 booting because the interface is twice the speed of USB. It's still surprisingly snappy even on USB 2.0. So does anyone have any experience with this enclosure? Does the Iomega FW800 interface just not support booting? Does El Capitan support booting off Firewire? Would downgrading my OS help?