dual boot OS Monterey and El Capitan

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Level 1

8 points

dual boot OS Monterey and El Capitan

I've just bought a 2015 iMac 17,1 running OS Monterey, and want to partition it to have a second volume with El Capitan - to be able to run some older legacy software, Aperture etc.

I've been using an older 2010 iMac that I set up as dual boot with El Capitan and Snow Leopard, so I'm familar with the basics of how that works, but I'm unsure about the implications of Monterey running under APFS, and El Capitan in HFS+, and whether that's still possible, or not a problem. I've read that in APFS you can have separate OSs on separate volumes without creating a second partition but I don't really understand the implications of that- and not sure if that would work for this setup? I'm also wondering if it'd be better overall to simply keep the new Mac running Monterey only, and have El Cap on a separate external SSD, and so avoid any potential conflicts with different file systems.

Any advice greatly appreciated, thanks.

iMac Line (2012 and Later)

Posted on May 16, 2022 10:56 AM

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Answer 1

Top-ranking reply

PRP_53

Level 10

91,483 points

Jun 13, 2022 3:47 AM in response to boris392

Worth a read ?

Portion from link below

To install a version earlier than the version currently installed, use a bootable installer of macOS High Sierra or later.

Perhaps your second option of El Capitan on Separate External Drive will present less complications.

Use more than one version of macOS on a Mac

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Answer 2

ivan54

Level 4

2,858 points

Jun 12, 2022 2:59 PM in response to boris392

Howdy,

You could partition your internal hard drive to run both El Capitan and Monterey from the same drive, however you would need to back it up and partition it with Disk Utility from El Capitan as Mac OS Extended volumes first. That would allow you to install El Capitan on one of the volumes, and when you installed Monterey on the other partition it would be converted to APFS.

You can go from Mac OS Extended to APFS, but not APFS to Mac OS Extended. El Capitan doesn't recognize APFS.

But you mentioned an external SSD. I would highly recommend that.

I have an iMac 15,1 (Retina 5K 27" Mid 2015) and purchased a 2TB SK hynix Gold P3 PCIe NVMe Gen3 SSD, put it in a Crucial enclosure and it breathes new life into the iMac. I avoid booting from the internal hard drive as much as possible. Leave your internal drive for emergencies or testing.

Observations of my setup.

I wanted to boot from a variety of macOSes so partitioned the 2TB SSD into 6 partitions.

On my very old Mac Pro, I installed El Capitan onto a 32GB Flash Drive and copied the Install OS X El Capitan.app to it.

That allowed me to boot the iMac with El Capitan, run Disk Utility, partition the SSD and install El Capitan onto one of the partitions, all from the Flash Drive.

During the El Capitan install I got an error that the installer can't be verified and may be corrupt.

To get around that and avoid the verification check, I disconnected from the Internet (disconnect Ethernet, turn off WiFi) and set the Date to a date close the the release date of the OS.

Remember to Save the date change.

I was then able to install the other macOSs I wanted.

How to get old versions of macOS

Note some of the downloads are upgrades so I had to install El Capitan on the partition first then upgrade to the newer OS.

Here's my current SSD partitions.

Note some partitions are Mac OS Extended and some are APFS.

(Catalina, Mojave and Big Sur are APFS).

And booting from different OSes I may or may not see some of the various volumes, or get an Incompatible Disk error, which I just ignore.

That's all fine. I can boot from whatever I want and booting from the SSD is much faster than the internal drive.

Of course you could just install El Capitan on the External SSD and be done with it :-)

ivan

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Answer 3

boris392 Author

Level 1

8 points

Jun 12, 2022 11:39 PM in response to ivan54

Hi, thanks for taking the time for such a detailed reply! I ended up doing more or less exactly what you suggest.

First off I just installed El Capitan on an external SSD and ran it off that perfectly fine.

I also decided to downgrade the iMac's internal drive (which is also an SSD ) from Monterey back to Mojave, as I was a little nervous about jumping up all the way from El Capitan straight to Monterey with all my files. So then I thought I may as well partition the drive at the same time, and install first El Capitan on one partition and then Mojave on the other, just to see if that works, and it turns out that worked fine too, and that's how I'm now using it. I haven't yet re-upgraded form Mojave to Monterey, but plan to do that next.

This set-up seems to work really well, and the only reason I can think of why I might want to revert to running El Capitan from the external SSD instead would be if I needed all the space on the internal SSD for the current OS and files - which right now I don't.

I'm interested in why you say you avoid booting from your internal HD as much as possible though?

Thanks, Boris

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Answer 4

ivan54

Level 4

2,858 points

Jun 13, 2022 8:50 AM in response to boris392

Glad you got it working the way you want.

My iMac has a spinning platter internal hard drive and it's much slower than an SSD.

As your internal drive is an SSD, then my experience doesn't apply to you, booting from the internal.

Enjoy,

ivan

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Answer 5

boris392 Author

Level 1

8 points

Jun 14, 2022 2:57 AM in response to ivan54

Ok that makes sense - thanks again, I really appreciate your taking the time. It's a big relief to get confirmation from someone who knows what they're doing that I set it up correctly, as I still wasn't certain I wasn't creating problems further down the line!

All the best, Boris

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