Partition Mojave HD to run Sierra

Started 2 months ago | Discussions thread
JPAlbert Senior Member • Posts: 1,065
Re: 2 things come to my head

silentstorm wrote:
"You cannot install an older OS once a newer version is there. Not even onto external drive. "

This is absolutely, positively UNTRUE and is misinformation.

You CAN install an older version of the OS on a Mac, so long as it shipped with that version of the OS.

This is the reason that Apple offers a "special version" of internet recovery, accessed through "command-OPTION-SHIFT-R" at boot.
When you boot to this version of internet recovery, and it will download and install the ORIGINAL OS that shipped with the Mac you're using (or the current iteration of that particular OS). You DO have to completely erase the drive in the Mac beforehand to reinstall on the internal drive.

I just checked this with the 2015 MacBook Pro I'm holding in my hands and typing on right now. This MBP shipped with OS 10.11 El Capitan, but is now running OS 10.12 "Low" Sierra.

The steps I took:
1. Booted with the Command-OPTION-SHIFT-R key combo into special internet recovery (it takes a while).
2. I plugged in an SD card I had (which was already formatted to HFS+) and let its icon mount on the desktop.
3. Internet recovery presented the "OS utilities" to me.
4. I choose "reinstall OS X"
5. I plugged in an SD card I had (which was already formatted to HFS+)
6. The OS installer opened -- Install El Capitan.
7. I began clicking through, and selected the SD card as the target (which would be "an external drive")
8. The OS install actually began, but at that point I pressed and held the power button to cut off power. But again, the OS install had begun.

Again... you CAN install older versions of the OS onto any Mac which was originally capable of RUNNING those versions of the OS.

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