Two macOS's one iMac

[venique]

Hello.

I have an iMac 27" late 2013 with HDD with macOS Catalina and free NVME slot. I'm planning to fit it with NVME SSD with adapter and install macOS Monterey through OpenCore Patcher on it. All my programs and data will be left within old HDD. As I want clean and fresh install I don't want to migrate them with some kind of software or TimeMachine and will re-install them manually time by time. And it will be more convinient if in that moments I will be able to boot from old HDD to check what exactly I need to transfer.

So my question is - what will happen if I will boot into macOS Catalina and then into macOS Monterey again and again inside one hardware? Will some software conflicts happens? Maybe because of different bootloader etc? If nothing will happen, maybe it's also okay to connect old HDD to MBP 2019 with Big Sur and boot from old HDD there? Or in this case it definitely will crash on loading?

 

[OrenLindsey]

Here's an Apple support page that might help:

Use more than one version of macOS on Mac - Apple Support

If you install a compatible macOS on an additional internal or external storage volume, your Mac can start up from either volume.

support.apple.com

 

[Bigwaff]

I have an iMac 27" late 2013 with HDD with macOS Catalina and free NVME slot. I'm planning to fit it with NVME SSD with adapter and install macOS Monterey through OpenCore Patcher on it. All my programs and data will be left within old HDD

If your are going to crack that thing open, why leave the slow, aging HDD in there? Replace with an SSD as well.

 

[Juicy Box]

So my question is - what will happen if I will boot into macOS Catalina and then into macOS Monterey again and again inside one hardware? Will some software conflicts happens?

I have many Macs with multiple boot volumes of different OS versions. I haven't ever noticed an issue. That said, I haven't tried it with Monterey and Catalina, but I don't think there would be any issues.

Like @Bigwaff said, why not replace the SATA HDD with a SATA SSD while you have it open. SATA SSDs are really cheap now and you don't want to keep that old HDD inside if you don't have to. An aging/failing HDD can cause weird boot issues and kernel panics even if you are using another drive to boot from.

Another thought, the PCIe slot on the Late 2013 iMac is limited to 1GBps (PCIe 2.0x2, not too much higher than what you can get from a decent SATA SSD. SATA SSDs are cheaper and a lot easier to install on the iMac than replacing the blade.

 

[OrenLindsey]

If your are going to crack that thing open, why leave the slow, aging HDD in there? Replace with an SSD as well.

Yeah this is a good idea too

 

[venique]

Thank you all for advices!

I'll keep in mind that I can boot to different versions of macOS without any issues. I was afraid that each of macOS flashes BootROM\EFI\Firmware or something like that, which will match only exact version of macOS, and that can cause some issue while booting to other version that awaits different BootROM etc.

And probably I'll replace old HDD to SATA SSD with cloning macOS via SuperDuper. It seems that in future after full migrating to NVME SSD I'll keep it as second drive for downloads and other unimportant stuff to keep NVME more healthy.

Yeah, it will not gain me much more speed in comparing to SATA but as it was said - if I'm going to open iMac I need push it to the limits. I'm also planning to install full RAM package, upgrade CPU, renew thermal interface and install thermalpads, clean dust from display, even change the battery on motherboard. In other words - rebuild from scratch.

 

[Fishrrman]

You can boot back-and-forth from one version of the OS to another.
I've done this for many, many years.

Instead of splitting open the iMac, have you considered just adding an external USB3 SSD to be "the alternate boot drive"?

 

[venique]

Thank you for sharing your expirience. I didn't considered about external bay because my iMac requires heavy cleaning so I need to split it anyway.