Late 2009 iMac reinstall OS without discs

I don't think you need a Mac of that vintage to download the installers Apple has up for versions earlier than High Sierra. The older installers are a direct file download.

How to download and install macOS - Apple Support

I agree, however, that this is going to largely be a labor of love because a 2009 iMac is very marginal in the modern world. High Sierra is still usable but probably not for much longer since many major apps such as browsers that do still support it will be stopping doing so later this year.

Once you have El Capitan or something on it, you can then upgrade it using the installer that does need an OS on the computer to work.

FYI, the late-2009 iMac can run macOS 10.13 High Sierra.

Discs... you'll be looking for factory grey discs specific to the late-2009 iMac. Hard to find and easy to buy the wrong ones. OR you're looking for the retail Snow Leopard 10.6.3 disc, white face with the cat.

You can try to install 10.11 El Capitan via a bootable usb installer, but you'll need a working Mac of that vintage to download and create the installer. That will be a Mac built between 2007-2015.

Create a bootable installer - Apple Support

To be honest, my friend, the old iMac really isn't worth more than a few pennies today. Rather than spend time and energy on finding an OS to reinstall, I believe you'd be wise to simply write this one off and recycle it or sell it as-is for parts. Truth be told, it's really only worth that.

It probably came with Snow Leopard discs which may be very hard to find now. Apple won't have them and if you get them on the used market might need to make sure you get the specific version for that computer. I don't know the exact model you have but this web site says it came with 10.6.1 which was the base level Snow Leopard. It is just possible that the retail upgrade disc may work.

Another option is --> Create a bootable installer - Apple Support You can't install High Sierra but you can install a lower macOS.

Limnos wrote:

It doesn't say you cannot create a bootable installer. It appears to be designed so you can create a bootable installer for any macOS version in that class of older OS versions that don't need to be installed on the computer itself.

No, the documentation may not say that in so many words (maybe it does somewhere), but in practice this seems to be a fact.

We know that Apple do not always paint the "whole picture" so to speak in their documentation.

It is my experience that one cannot create a bootable installer for an OS that the Mac being used to create that installer cannot run.

I just attempted to create an installer for High Sierra using a MacBook Air M2. I was unsuccessful. I can create installers for Ventura and Sonoma.

I am successful using the same High Sierra installer app download on an Mac mini 2012 (Intel) that qualifies to run High Sierra.

This limitation isn't uncommon and fits right in with "a Mac cannot run a version of macOS that predates its release".

"You don't need a bootable installer to upgrade macOS or reinstall macOS, but it can be useful if you want to install macOS on multiple computers without downloading the installer each time, or you're unable to install a compatible macOS from the Finder or macOS Recovery."

And while "You don't need a bootable installer to upgrade macOS or reinstall macOS...", this is true if the problematic Mac can otherwise boot to Recovery or to Finder on its proper startup drive. Otherwise a bootable installer in one form or another (reinstall discs, usb stick) is a must have.

I recently installed High Sierra on a computer with Mavericks. I got a cannot install message and found that I had not partitioned using GUID which HS requires.

While yes, you can download the installer app with a newer Mac, that Mac cannot create the bootable installer for the older OS. And certainly the OP's 2021 MacBook Pro (M1 Pro/Max) cannot create the bootable installer for that '09 iMac.

So without having access to the original reinstall discs, a retail disc, an external boot disk or a way to create a bootable installer, that 2009 Intel iMac is not going to be restored.

It doesn't say you cannot create a bootable installer. It appears to be designed so you can create a bootable installer for any macOS version in that class of older OS versions that don't need to be installed on the computer itself.

"You don't need a bootable installer to upgrade macOS or reinstall macOS, but it can be useful if you want to install macOS on multiple computers without downloading the installer each time, or you're unable to install a compatible macOS from the Finder or macOS Recovery."

Thanks. I'll take note of your experiment. :)

And I'll be happy to learn I'm wrong... but not always. 😃