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OS X - Operating System General OS operation information and support

For iMac 27" Owners - How to downgrade from lion to SL

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Member Since: Apr 13, 2012
Location: Switzerland
Posts: 2
dyl is on a distinguished road
Mac Specs: iMac 27", 10.6.8, 3.4 GHz Intel Core i7, 16 GB RAM

dyl is offline
Hello everyone

Since the purchase of my iMac 27" on the 22.02.2012, i have been stuck with lion and frustrated to the bone, however, i read so many posts in different platforms and non of them didn't help as they were mostly for MBP owners but then i decided to taste the waters myself.

Anyway, the following method worked for me and hope it will work for you too but at your own risk..!

Below are the steps how to complete remove Lion from your iMac 27" and install SL 10.6.3 ( yes, 10.6.3 )

I purchased my iMac 27" with Lion pre-installed and i can confirm it works, my Firewire 800 port is working, i can install all my programs and run without issues but i am not sure about the Thunderbolt as i am not using it.

Requirements / what you need.

1. Your current iMac 27" running Lion 7.x.x with a firewire input.
2. Another Mac, MBP or another iMac running SL and must also have a firewire input too.
3. Snow Leopard install DVD ( 10.6.3 in my case)
4. Firewire 800 / 400 cable


1. Download Snow Leopard 10.6.8 Combo and the Supplement update from Apple using the machine running Snow Leopard from Apple - Support - Downloads
2. Back-Up all your important documents on your iMac to an external disk before proceeding.
3. Shutdown your iMac when you have done the back up.
4. Connect the Firewire Cable 800 or 400, whichever is supported to both machines.
5. Insert your SL install DVD in SL Machine NOT the iMac 27".
6. Start your iMac 27" and press T during startup, this will start your iMac 27" in the Target Disk Mode.

Now your iMac disk and any other disks in it should now be showing on the SL Machine desktop as an external disk/s.

7. Make sure you inserted your SL install Disk into the SL Machine ( current running )
8. Click Install Mac OS X
9. Click on Utilities NOT Continue then click on Restart.

NB: Your iMac 27" should still be in the Target Disk Mode, do not switch it off

10. After restart, select English or your Language of preference and click the next.
11. DO NOT press CONTINUE, instead look at the top Menu and Click on Utilities.
12. On the drop down menu, click on Disk Utility

Look for the Disk connected with a Firewire cable and click on it ( the MAIN DISK not the CURRENT PARTITION ). You should be able to see First Aid, Erase, Partition, RAID and Restore.

13. Click on Partition

Now you can see your disk partition window and on the right the name as well as the format of the disk.

14. Click the (-) button at the bottom of the window to delete the partition and confirm you want to delete it.
15. Create a new or multiple partitions using the (+) , remember to use choose Journaled and check the options that they are set to Default before formatting.
16. Name the top Partition ( Macintosh HD or just HD like i often do ), this will help identify the disk during the installation.
17. Format the disk and quit Disk Utility to continue with your installation.

Your installation will start, follow the instructions and remember to select the correct disk on the next step..!

Depending on your machine, you will have 20 min time to call granny or take your dog for a short walk.


18. After the installation is done, the machine will boot into 10.6.3 which you just installed on your iMac 27"

Now you need to copy the 10.6.8 and the other downloads to your iMac 27" Download folder.

19. Go the System Preferences and change the Startup Disk back to SL Machine ( the machine you are using )

20. Reboot the machine ( Now your iMac disks should be present on your SL Machine Desktop )

21. Open the Download Folder of your SL Machine then navigate to the Download Folder inside the Macintosh HD / HD on the desktop.
22. Copy the 10.6.8 Combo files as well as the supplement files to the Macintosh HD / HD Download Folder.

When done, Change the Startup Disk to 10.6.3 ( which is your new installation )

23. Reboot the machine ( you should now be in 10.6.3, iMac 27" )

24. Now install the 10.6.8 Combo update files from the Download Folder to update to 10.6.8 ( if you don't do this and try to start your iMac with 10.6.3, it won't start and will beep endless )

If you are done,

25. Go to System Preferences and change the Startup Disk back to SL Machine otherwise you are not able to boot into your SL Machine afterwards.
26. Shutdown both machines and disconnect the firewire cable.

Reboot both and you should be up and running.

In a case your SL Machine prompts you for a password, you forgot to change the Startup disk. To solve this, just reconnect the cable and play with your startup disks and this should resolve it.

Done !

Be aware, it takes a few more seconds to start than usual but who cares..!

No more stupid Lion on your machine.
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Member Since: Jan 23, 2008
Location: Fort Worth, Texas
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Mac Specs: 27" iMac i5, 3.2 GHz, iPad 3, iPhone 5c, 3 iPods, Yosemite

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To anyone who wishes to try what member "dyl" posted:

I do not recommend downgrading any machine from Lion to Snow Leopard that came preinstalled with Lion.

It may void your warranty (I don't know this for sure.) I would also be concerned about any future firmware and driver updates that are specific for the particular machine you downgrade.

And as member "dyl" stated "do it at your own risk".
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Member Since: Apr 13, 2012
Location: Switzerland
Posts: 2
dyl is on a distinguished road
Mac Specs: iMac 27", 10.6.8, 3.4 GHz Intel Core i7, 16 GB RAM

dyl is offline
If your machine is under warranty i would suggest you don't do this, i have two private owned machines and if anything happens to the other, i am still able to carry on with my work. With 10.6.8 running on my iMac, i do not intend to make any adjustments whatsoever as it is only dedicated for video editing and all my would be supported programs in Lion are also running. However, for those who fancy all the gadgets on their system it is best you read this article Don't install a version of Mac OS X earlier than what came with your Mac to see what may not work or function properly after messing up your machine.
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Member Since: Dec 15, 2004
Location: USA
Posts: 614
ByzantineRemnant will become famous soon enough
Mac Specs: MacPro, MBP C2D, iMac G4

ByzantineRemnant is offline
Thanks for this post! I can confirm that I just succesfully downgraded from 10.8.1 to 10.6.8 from a refurbished 27" iMac I just purchased from Apple. In regards to "chscag," I agree that downgrading to a machine that did not originally ship with 10.6.x would be foolish. However, since the 3.4 i7 27" iMac I just purchased came out in May of 2011 a couple months before 10.7, I don't see there being any hardware issues as all of these components were OEM shipped from Apple to work with 10.6.6. Thank you for the detailed post "dyl," it was a nice step by step process. I do know how to target disk mode, but having a set of instructions was helpful.
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