I have a 2012 cMP 5,1 3.33 6-core with original 5770 GPU. Essentially I would like to try upgrading to Mojave (on a fresh boot SSD) but keep an older boot SSD, with an older OSX (eg Sierra or El Capitan), to use old software. I've bought a Sapphire Radeon RX580 Pulse 8Gb GPU to use with the Mojave boot SSD. 1) If I update the firmware to MP51.0089.B00 (and subsequently 184.108.40.206.0), can I still use an older OSX eg El Capitan or Sierra? Or am I permanently forced, on this cMP, to use Mojave from now on? 2) Where exactly is the new firmware written to, the CPU? Can I roll back to an older BootROM firmware if i have problems? If yes, how do I backup my older BootROM firmware or where can I download older versions? Thank you.