Clean install: OSX - iMac 2008

The last version of macOS that supports the 2008 iMac is macOS El Capitan (10.11.6). Once you have purchased a retail Snow Leopard (10.6.3) DVD from eBay, install from that DVD, and continue to apply software updates through 10.6.8, including the Mac App Store updates.

Then you can obtain macOS El Capitan from How to download and install macOS - Apple Support and refer to the steps in the Use a web browser for older versions section. The age of the Snow Leopard Safari browser may cause you issues here.

Understand that macOS El Capitan uses extremely old security features and most secure websites may block your Safari browser because it is using deprecated security protocols. Same drill for most current email servers and certainly financial institutions. No mainstream browser vendors provide current versions of their products for El Capitan now.

The boot blocks on that iMac are now Linux. Don't expect the drivers in Linux to provide comprehensive support of that iMac. Even after macOS is installed, that iMac may attempt to boot only to Linux.

Follow VikingOSX's instructions to the letter. You may need to boot from that retail Snow Leopard DVD and use its version of Disk Utility to erase the OS X boot partition first. Then, proceed to install Snow Leopard, upgrade it to OS X 10.6.8 using Software Update (which incorporates the App Store) and then use the App Store to download and install El Capitan.

Result:

A separate partition boots Snow Leopard.

It may interest you to know that Mac also boots Ubuntu Linux. I did not install Linux Mint but I imagine it's similar.

Having said all that... you will almost certainly be better off running Linux on that Mac. I am composing this message using Firefox since it's the only popular browser that continues to work with El Cap:

El Cap's version of Safari will no longer work with this site (and many others). You'll run into similar problems with email... etc.

Use a white Retail Snow Leopard 10.6 Upgrade DVD.

Update to 10.6.8 using > Mac OS X 10.6.8 Update Combo v1.1 - Apple Support

Then go to the Mac App Store and download the latest available macOS for that model.

Purchase a Retail Lion 10.7 USB flash drive.

see > https://www.ebay.com/itm/204762960880?

Then go to the Mac App Store and download the latest available macOS for that model.

Create a bootable El Capitan Installer on a USB flash drive.

see > Create a bootable installer for macOS - Apple Support

That Mac is too old for OS X Internet Recovery: Computers that can be upgraded to use OS X Internet Recovery - Apple Support

Attempts to boot Internet Recovery will result in booting the internal Recovery partition, assuming it still exists, which seems doubtful given the actions described.

John Galt wrote:

That Mac is too old for OS X Internet Recovery: Computers that can be upgraded to use OS X Internet Recovery - Apple Support

Attempts to boot Internet Recovery will result in booting the internal Recovery partition, assuming it still exists, which seems doubtful given the actions described.

Yes, but the Apple documentation for Recovery Mode at one time mentioned that using the Command + Option + R combination would allow a person to install the OS without needing the AppleID authentication. I'm assuming that was a special option for the older Macs which did not allow for Internet Recovery Mode & only had a local recovery partition so that those users which had "purchased" the earlier versions of macOS through the App Store would not need to tie a clean install to their AppleID when transferring a Mac to another person. Apple's single sentence did not clarify or elaborate. Unfortunately I don't have such a Mac to test and nobody has ever updated their threads to confirm that sequence worked.

You can try booting into Recovery Mode using Command + Option + R which may allow you to access the installer without needing to authenticate with an AppleID at least according to the Apple documentation.

This is correct... but only for Macs capable of it, and that Mac isn't. Hence the screenshot showing the exact model.

That is also its original hard disk drive. For a time I considered upgrading it to a SSD, but it still works as well as it ever did, and as time passed the need for one diminished. Macs stubbornly refuse to die. Unfortunately the world moves on and they just become functionally obsolete.

Just as unfortunately, Apple has made things difficult to reinstall OS X on Macs that did not ship with discs. Some of them were caught in the period of time you alluded to. Apple essentially abandoned those users — an ignoble first in Apple history.

Replacing or erasing the hard disk drive obviously wipes out its Recovery partition. Reinstalling OS X from disc is required as VikingOSX wrote. Upgrading it (to Lion or later as I recall) re-creates that Recovery partition (so did restoring a TM backup for that matter). However, this is not something I was willing to confirm since I already know obtaining those earlier OS versions has become problematic.