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I own an iMac that was recently updated in error by a family member (I've forgiven them). This was a disaster because some of my hardware and software no longer work! Previously this iMac was running Snow Leopard 10.6.8. So, I decided to install Snow Leopard again. I have the original disks (remember those things) that came with the computer but cannot install Snow Leopard. I have wasted two whole days on this and am now desperate.

My specs are:

  • Late 2009 iMac
  • 27" model
  • 16GB RAM
  • Apple Wireless Keyboard
  • Magic Mouse
  • Mac OS X 10.11.6 installed :(

What I have tried:

  • Booting into Recovery Mode, from both the local HD and internet, but my Mac doesn't seem to support this?
  • Installing Snow Leopard from original Apple iMac grey disks that shipped with the computer
  • Installing Snow Leopard from a retail version of Snow Leopard (borrowed from a friend)
  • Verified the optical drive is working fine (it reads CDs/DVDs and burns DVDs fine)
  • Verified that booting from the optical drive is fine (booted from a Prosoft Data Rescue II CD)
  • Followed the steps outlined at http://www.makeuseof.com/tag/upgrading-your-mac-to-snow-leopard-using-your-usb-drive/ to create a bootable installation USB. I did this in case there was some other issue with using the optical drive.
  • Searched Google, Apple discussions and here for an answer

I am willing to try anything at this stage. I have a function on February 19 to prepare for and I require Snow Leopard for compatibility with some specialized software/hardware.

Pleeeeeeaaase can someone help me? Pretty please with sugar on top…!!! :)

  • What happens when you try to use the SL Install DVDs? Does it show an error? Do you have a backup of your computer/important files? – At0mic Jan 25 '17 at 4:10
  • Are you near an Apple Store? If you take it to the Genius Bar, they'll help you install Snow Leopard and help with any problems you might run into. – NoahL Jan 25 '17 at 4:12
  • Both install disks won't boot. I have the original disk that came with the iMac and a retail version (borrowed from a friend) and the computer won't boot from them! Grrr. But just when I think it's the optical drive, it boots fine from a Prosoft Data Rescue II CD. Crazy, hey? Yes, I have backups, so have no problem wiping the drive if necessary. But if I can't boot from the original discs nor the bootable USB, how do I install Snow Leopard? This is my conundrum. – user221458 Jan 25 '17 at 4:15
  • I am 1.5hrs away from an Apple Store. I think I will have to do that if I can't get this sorted in the next few days. – user221458 Jan 25 '17 at 4:17
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    @Kent Actually, Late 2009 iMacs are compatible with macOS Sierra, although that begs the question as to how El Capitan was installed and not Sierra? – Monomeeth Jan 25 '17 at 6:58
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Depending on when you bought your iMac, it came pre-installed with either Mac OS X Snow Leopard 10.6.1 (10A2155) or 10.6.2 (10C2234). It is one of these versions that will be on the original discs you refer to and, based on your lack of success with the retail copy, it is also one of these versions your friend must have as well.

Now, the reason your iMac isn't booting from any of these is that it can no longer run any version of Mac OS X before Mac OS X Snow Leopard 10.6.3.

Why?

Well, the key issue here is that your iMac is one of the quad core models. These were options for the 27" model only and both variants (i.e. the i5 and i7 versions) had a firmware update. Because of your music requirements (as expressed to me via Comments), I am 99.9% certain you installed this firmware update (which Apple recommended all users do anyway).

It is this update that is preventing you to boot from your Snow Leopard discs because the firmware update itself requires as a minimum Snow Leopard (10.6.3). The installers you're trying to use are for Snow Leopard 10.6.1 or 10.6.2.

Solution

IMPORTANT: This solution is based on your comment that you have backups of all your files etc. Proceeding with this will reformat your hard drive - so proceed carefully and ensure you have a current backup!

You can use Target Disk Mode to get around your problem. More specifically:

  1. Borrow your friend's MacBook Pro and ensure it and your iMac are both switched off
  2. Connect the two computers together via a FireWire cable
  3. Start your iMac in Target Disk Mode by holding down the T key on your iMac keyboard immediately after starting up. Keep the key down until you see the icon floating around the screen.
  4. Start the MBP
  5. Once booted, your iMac's hard drive will appear on the MBP desktop just like any external hard drive would.
  6. Insert the Snow Leopard retail disk into the MBP and follow the prompts to install Snow Leopard. Ensure you select the iMac hard drive as the install location - DO NOT select the MBP hard drive or you'll have a very unhappy friend!
  7. When the installation is complete, reboot the MBP again and immediately hold down the Option key. When prompted to select a statup disk, select the iMac's drive and hit Enter.
  8. You're now booting the MBP from your iMac hard drive.
  9. Once booted, use the Apple Menu > Software Update option to install updates.
  10. Keep installing updates until you have at least Mac OS X Snow Leopard 10.6.6 (the first version which includes the Mac App Store).
  11. Once you're running Mac OS X 10.6.6, shut down the MBP and switch off your iMac.
  12. Disconnect the FireWire cable from the two Macs.
  13. Reboot your iMac and update/install whatever software you need to get it working as you want.
  14. Copy/migrate across any files etc from your backup.
  15. Crisis averted!

Obviously the above steps will take some time, but let me know if you run into any problems.

As an aside, if you don't have access to a FireWire cable, or your friend doesn't want to lend you their MBP, you could try Kent's suggestion in the comments. In theory his suggestion should also work.

  • Sir, this looks very promising indeed!!! I have a Firewire cable and my friend is dropoing her MacBook Pro around to me shortly. I will do this tonight! – user221458 Jan 25 '17 at 9:23
  • I've reinstalled tons of 10.6 onto mostly MacBooks using either the 10.6.0 or 10.6.3 retail discs but never encountered that firmware problem. It's specific to the 27" model? And in step 6, erasing the iMac hdd is also an option. From experience, mail and other things upgrade from 6->11 but not the other way around. So you're better off starting with a clean slate from your backups. – Kent Jan 25 '17 at 12:44
  • @Kent You're exactly right - upon re-reading Step 6 I see I didn't actually specify that erasing the drive was the option the OP should take. I guess my Important note (before the steps) mentions this, so hopefully the OP does just that. And yes, the firmware issue is specific to quad core models (which was only a built to order option on the 27" models). – Monomeeth Jan 25 '17 at 13:31
  • You, Sir, are a scholar and a gentleman! My iMac is back on Snow Leopard and most importantly my specialized software and hardware works again!!! Thank you!!! You have my gratitude! :) – user221458 Jan 25 '17 at 22:28
  • A scholar and a gentleman? Well, thank you! :) More importantly, I'm glad you're back on track for your function. Good luck! – Monomeeth Jan 26 '17 at 0:28

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