Partition Mojave HD to run Sierra

Started 5 months ago | Discussions thread
Tom_N Forum Pro • Posts: 19,353
Re: 2 things come to my head

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.

Right.  You can't install an older OS (that was originally supported on your model of Mac) over an old OS, but you can do a clean install on a freshly-reformatted drive/volume.

If you are going from a modern version of macOS that supports and prefers APFS, to an older version of macOS that does not know how to handle APFS, you probably will also need to redo the formatting on the target drive to use HFS+ (Mac OS X Extended) rather than APFS.

Even if you are going from a modern version of macOS to an earlier one that supports APFS, you may want to redo the formatting on the target drive.  APFS is a work in progress and some of the older implementations may not understand all of the options that APFS filesystems created using newer versions of macOS use.

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).

It sounds like knowing to use the "special" Internet Recovery system instead of the "usual" one may be an underappreciated key to getting things to work.  Good tip.

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