If this is the case then one way I can think of to revert the firmware is to install the original 10.6.4 installer CD to revert the firmware done by that install. If installation changes some of my Mac Pro's firmware, presumably the stuff that affects the boot, do I assume that I cannot simply boot back my 10.6.8 once I've done one of these installs beyond 10.6.8?

'A bootable installer doesn't download macOS from the internet, but it does require an internet connection to get firmware and other information specific to the Mac model.'ĭoes that mean it changes my Mac Pro's firmware or it only accesses the appropriate firmware to add to the installer to allow it to boot on my machine? But what has me concerned is the line in that HT201372 doc (under Learn more near the end) which states: I haven't tried this as yet because my main concern is if I don't want to use the result how can I revert back to my 10.6.8? As I said I would do this to a copy of my 10.6.8 drive and ideally if I wanted revert I simply boot from the original 10.6.8. Then I can use that installer to upgrade (a copy of) my 10.6.8 to 10.11.

According to Apple's 'How to create a bootable installer for macOS' (HT201372) I can download the El Capitan package installer to install 'Install OS X El Capitan.app' into 10.6.8's /Applications, and then use its createinstallmedia tool to create a bootable 10.11 installer on to a flash drive. I want to upgrade my mid 2010 Mac Pro's 10.6.8 Snow Leopard to, say, 10.11 El Capitan (please don't ask why I haven't ever done this till now).

Mac os 10.6 8 upgrade to el capitan