Can I still run Yosemite on early 2015 MB Air with Mojave firmware?

[xaosnox]

I've been running Yosemite for hardware support and aesthetic reasons. (I can theme it so it doesn't look like it was made by color blind slave labor children.) But, Mojave looks livable, and I'm tired of Yosemite's networking issues, so I'm ready to attempt the leap.

I intend to install it on an external drive initially, as it will take me a week to test all of my audio plugins and get it set up, and I need to still be able to run Yosemite from my internal drive while I'm working on getting Mojave working.

I'm concerned that if I do the firmware updates, I might not be able to boot into Yosemite. If this is the case, I'll probably need to make a Virtual Box of my existing system.

Is there anyone here who has tried running Yosemite and Mojave in a dual boot situation?

Thank you all for your help!

 

[Intell]

You can still run 10.10 on a system that was updated to 10.14. Just note that your 10.10 OS will not be able to read your 10.14 OS because it will be formatted as APFS.

 

[xaosnox]

I'm reading that it can install on macOS Extended Journal partitions, too.
https://discussions.apple.com/thread/8553632

Does anyone know if I have to do anything special during the install to tho do that other than have it install on a pre-formatted partition?.Seems like if I want to install it on an Extended Journaled partition, I might have to install Yosemite on it first, then upgrade that install. I don't want my whole hard drive formatted as APFS because I share files with an old G5 server. (15 years old and still going strong! Wish they made them like they used to.)

I'm keeping this in the same thread because it's relevant to the original topic.

 

[Intell]

10.14 will not easily install onto a HFS+ partition. If you do get it onto one, you will not be able to install any updates once it is installed. I strongly recommend installing it onto an APFS formatted drive. If it is APFS, you'll still be able to share files via SMB, but not AFP