Partition Mojave HD to run Sierra

Started 3 months ago | Discussions thread
ChrisLumix Veteran Member • Posts: 3,361
Re: 2 things come to my head

silentstorm wrote:

JPAlbert wrote:

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.

Oh please Albert. Read my reply again.

For convenience... here is the part again

"Especially true if the hardware comes with the newer OS, the firmware in the hardware will block any OSes that are older than the shipped version."

What you said is going back to the original OS that shipped with the hardware. You can't go back older than the shipped version.

If OP MBAir comes with Sierra, then yes, he can reformat his drive and get back the original OS. But if it comes with Mojave, then there's no way he could run Sierra on it, the firmware will block out Sierra.

EDIT:

I just checked, OP MBA laptop is a 2017 model. If it comes installed with Hi Sierra (original shipped OS), then he can't go to Sierra at all.

That's only partly true. My original 2011 iMac shipped with Lion. The supplier was able to erase and reformat the HD and install Snow Leopard on it. That model (not that machine) originally came with Snow Leopard, though later sales were shipped with Lion.

So the "shipping" point is not as moot as which particular OS the model in question FIRST SHIPPED WITH. Later examples may ship with a later OS but that doesn't mean the original OS for that model can't be installed.

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Panas0n!c Lum!x LX100, TZ60

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