What’s the oldest OS I can put on a 2015 iMac?

[stinkhorn9]

Big Snow Leopard fan here - still rocking my 2009 Mac Mini and MBP (unsullied since day 1).

Recently I factory-reset my mum’s old 2015 iMac and - ta daaa - now it has Big Sur on it, and I’m wondering whether it’s possible to ‘downgrade’ the OS, and if so, to what extent? I’m hoping greater compatibility with the software I use - and possibly even better performance through running a less-whizzy OS - might be in the offing...

 

[ssmed]

You don't say which model. The Apple specification pages e.g. https://support.apple.com/kb/SP733?viewlocale=en_GB&locale=en_GB state the software that was originally loaded. Sometimes if an update is imminent when the computer is released you need the OS version after the one stated.

In a connected world I would not be so keen to have an unsupported software. However, Apple could support security updates for longer periods as the hardware is often doing well for a long period, but that is not a good business model.

 

[profcutter]

The oldest OS you can install on a machine is usually the OS that was available when the machine first came out. Maybe El Capitan or Yosemite?

 

[joevt]

I have 10.4 Tiger working on my Mac Pro 2008 even though it came with 10.5 Leopard. It's missing proper support for my GPU (can't sleep or change resolution) but otherwise works fine.

Is there VM software that will let you use older versions of Mac OS X?
I think Parallels goes out of its way to not work with virtual or real disks that have old macOS versions. You have to use a server version of macOS or something... I haven't tried it recently though.

 

[stinkhorn9]

I have 10.4 Tiger working on my Mac Pro 2008 even though it came with 10.5 Leopard. It's missing proper support for my GPU (can't sleep or change resolution) but otherwise works fine.

Is there VM software that will let you use older versions of Mac OS X?
I think Parallels goes out of its way to not work with virtual or real disks that have old macOS versions. You have to use a server version of macOS or something... I haven't tried it recently though.

Thanks for your replies ? I think it shipped with Sierra- but I’ve been reading good things about Mojave- which seems to be the last OS available that would support 32-bit apps. Obviously I have no idea how to go about getting hold of an earlier OS - could someone talk me through it? Assume I’m dense (I’m dense, so it’s ok ??),

 

[joevt]

Thanks for your replies ? I think it shipped with Sierra- but I’ve been reading good things about Mojave- which seems to be the last OS available that would support 32-bit apps. Obviously I have no idea how to go about getting hold of an earlier OS - could someone talk me through it? Assume I’m dense (I’m dense, so it’s ok ??),

2015 iMac maybe came with El Capitan? It should support all versions of macOS since then.

You can try these instructions:
https://support.apple.com/en-us/HT211683

 

[stinkhorn9]

Thank you joevt ?