My Mac repeatedly beeps three times on startup. What does this mean?
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:


Boot your iMac into Target Disk Mode by pressing and holding the T key as you startup
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)
Boot the MBP from the Snow Leopard disc 
Now install Snow Leopard onto the external drive (i.e. the iMac’s drive)
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
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!