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When I try to boot my Mac I get nothing but strange beeps. They beep as follows:

BEEP BEEP BEEP [3 second pause] BEEP BEEP BEEP [3 second pause] and it continues until I power off.

So far, I have:

  • checked with Apple KB - About Mac startup tones and this pattern is not listed as an official form of communication.
  • rebooted a number of times and the same exact pattern occurs.
  • used a timer to verify it is definitely 3 seconds in between the 3 beeps.
  • booted into Apple Hardware Test and no problems found after running extended tests

Is my Mac possessed? Assuming not, what is it trying to tell me?

Background

For various reasons I want to re-install the original Mac OS X. To do this I am trying to boot from an original Snow Leopard disc in the SuperDrive. I have no reason to suspect a problem with either the disc (no scratches or other signs of damage and it boots a Mid 2009 MacBook Pro just fine) or the optical drive (it loads other discs fine).

The Mac boots fine otherwise (both normally and in Safe Mode), but just doesn’t want to boot from this disc and I have no idea why, but I’m pretty sure the beeps are the key! :)

Specifications:

  • iMac, 27", Mid 2011
  • 16GB RAM
  • 2 GB GDDR5 (Radeon HD 6970M)
  • 3.4GHz Intel Core i7
  • Mac OS X El Capitan 10.11.6 installed on the internal drive

Update

Upon checking after Monomeeth's answer I can add that the original Snow Leopard disc I am trying to boot from is indeed a retail version containing 10.6.3. enter image description here

  • 1
    "I will follow Monomeeth's advice and updated my question with the results." - Note that in general, you shouldn't edit the outcome of following an answer's advice into the question itself; instead, accept that answer (if it works) and leave your description of the outcome as a comment on that answer. – V2Blast Sep 6 '18 at 7:51
  • I had a near identical problem that was caused by - what seemed to be - loose ram. I unscrewed the back, disconnected the battery, took out the ram, inspected for damage (none I could see), flipped the positions of the cards for good measure, and it booted fine. One additional problem that I had had was that if the keyboard flex when moving, the computer would shut down or restart. This is for anyone who reads this and goes “my problem is so close but different” like me <3 – Throsby Jan 21 at 8:23
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No, your Mac is not possessed. And Yes, it’s trying to tell you something!

A looping three beeps in between three seconds during startup is your iMac’s way of telling you that the operating system you’re trying to boot into is incompatible with your Mac hardware.

In other words, the Snow Leopard disc you’re trying to boot from contains a version of Snow Leopard that is earlier than version 10.6.6 (the earliest your iMac can support).

My guess is that you’re not using the grey disc that shipped with your iMac. It’s either one that belongs to another Mac or it’s a retail version that’s earlier than 10.6.6.

The only ways to get around this is to use another disc or do the following:

  1. Boot your iMac into Target Disk Mode by pressing and holding the T key as you startup
  2. Connect it to your Mid 2009 MBP with a FireWire cable (once connected it’ll be seen by the MBP as an external hard drive)
  3. Boot the MBP from the Snow Leopard disc
  4. Now install Snow Leopard onto the external drive (i.e. the iMac’s drive)
  5. Reboot the MBP from the iMac drive and then upgrade Snow Leopard to the latest version (i.e. 10.6.8) so that you know for sure it’ll boot the iMac fine
  6. Shutdown both the MBP and iMac and then try rebooting the iMac as normal from its internal drive that now has Snow Leopard installed.

WARNING: You should always ensure you have a backup of your data, especially when performing operations such as this!

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    I'd be hesitant to recommend this—if it doesn't boot from the CD, is it really likely to boot from the disk? – Kevin Sep 5 '18 at 16:18
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    @Kevin: A similar process for installing Windows 10 on older iMacs works just fine (where Bootcamp is limited to Windows 8), so it might work okay. – user286843 Sep 5 '18 at 17:52
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    @Kevin Thanks for the comment. The beeping pattern described by the OP definitely means the OS isn’t compatible. Since they’re trying to install Snow Leopard, we know it must be version 10.6.0 to 10.6.5 they’re trying to boot from as their iMac does support 10.6.6 to 10.6.8. So, assuming there’s no other unknown issues, then following my steps will work as long as they remember to do Step 5 (as that’s what will update the installed version to a compatible one). Hopefully the OP will report back either way. If you have any other questions please feel free to ask. :) – Monomeeth Sep 5 '18 at 22:30
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    @user301113 Yep, it certainly fits! And thank you for coming back to share your results, it's sure to help others down the track! :) – Monomeeth Sep 6 '18 at 4:46
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    @Monomeeth You're a legend! I followed your steps and my Mac is now purring nicely on Snow Leopard. It was actually very easy to do. :) – user301113 Sep 6 '18 at 12:45

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