Bit of a techy OS X Mac question for some of you. I have an old Macbook Pro that I still use when I travel and for some location recording. It's from 2011 but it's a quad core i7 with 16GB ram and an SSD drive so fine to work on like that. I recently upgraded it to the latest Catalina operating system with a simple hack. All works well. There's something that was bugging me though with the thunderbolt port and I need to upgrade the thunderbolt firmware on the MB Pro. For any of you that are still reading this here's my problem. The firmware update can only be installed on an older version of the operating system ie Mountain Lion - So... - I made a bootable USB image of Mountain Lion, held alt on startup and can boot from the Mountain Lion image BUT how can I run the firmware update file? The options I have are things like restore time machine backup, install mountain lion, disk utility etc but no way to browse to find the file. I have manage to mount a separate usb key with the firmware update on but still can't run the file. I'm wondering if I need to use Terminal which I can use to navigate to the 2nd usb key with the firmware on and then run the file from that somehow? Thanks in advance if you can help.