Partition Mojave HD to run Sierra

Started 2 months ago | Discussions thread
JPAlbert Senior Member • Posts: 1,066
Re: Partition Mojave HD to run Sierra
1

OP:

I recommend that you DON'T partition the internal drive.
Not "for only one app".

Instead, get a smallish-sized USB3 SSD drive (256gb should be fine even 128gb might do well enough), and install a copy of Sierra onto that.

You can either buy a ready-to-use USB3 SSD, or get a "bare" 2.5" SATA SSD and an enclosure, something like this:
https://www.amazon.com/Inateck-Inch-Drive-Enclosure-External/dp/B00FCLG65U/ref=sr_1_3?dchild=1&keywords=inateck+2.5%22+USB3+enclosure&qid=1625363201&sr=8-3

For 10.12 "Low" Sierra, format it for HFS+ (Mac OS extended with journaling enabled, GUID partition format).

Then download a copy of Low Sierra from Apple, see this page:
https://support.apple.com/en-us/HT211683

I believe this comes as a disk image. Open the image, launch the pkg file (I'm going from memory), and it will put the Low Sierra installer into your applications folder.

Then launch the installer and "aim it" at the EXTERNAL SSD (NOT the internal drive).

Expect the Mac to restart one or more times, and the screen may go dark for a minute or two. Be patient.

When done, you'll be presented with the initial startup screen (choose your language). Follow through and set up a basic account. I'd skip iCloud, email, etc.
Then install the app you intend to use.

When you need to switch boot, connect the external drive and boot with the option key held down CONTINUOUSLY until the startup manager appears.
Then select the external drive and hit return.

I've been doing stuff like this for many many years...

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