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I have a Mountain Lion InstallESD.dmg I also have a iMac 2007 model which I used an old install disk (incompatible) to access it's disk utility. I removed everything from my internal HD and reformatted as Macintosh Extended Journalled

Then I stuck InstallESD.dmg onto a USB and plugged in a second USB.

I formatted the second as Extended Journaled (GUID Partition Table) and burned the DMG from USB one to USB two.

I then turned my iMac off and rebooted holding alt (option). It picked up my bootable USB and I clicked it. It went onto the grey screen with the apple logo (I have been told it's normal) however after a short period of time, the apple logo changes to a prohibitary sign and a spinning cog shows underneath. I left my iMac for a long time and nothing happened. What Am I Doing Wrong?

NOTE: I only have access to disk utility, terminal and the essentials that can be used straight from boot disk

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For reference:

Now to answer your question:

  • If your iMac is a 2007 Model (Early or Mid), the Configuration says its Core 2 Duo, so Mountain Lion should definitely run if you bought a legal version.
  • If it is not running, then you need to follow the reference link to making a USB bootable OSX installation, and make it again correctly. The way you have mentioned making it sounds wrong to me.
  • If both the above don't work, you need to check your iMac model again using the reference above, hopefully its not a G5 Power PC processor, although it cannot be since the last G5 iMac was released in 2006

I am sure, if you make you USB Bootable OSX disk again using the Tutorial above it should solve your problem. Because your iMac does have the Minimum Requirements for Mountain Lion. The earliest iMac it supports is the 2007 one. Good Luck!

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I followed the same tutorial as the one provided – Caitlann Lloyd Aug 24 '12 at 19:11
Are you using a Legal copy, or you downloaded it from somewhere? – aliasgar Aug 24 '12 at 19:13
Perfectly legal, took from Package Contents – Caitlann Lloyd Aug 24 '12 at 21:01
Package Contents? The minimal 400mb or 500mb InstallESD.dmg which comes with a pre-installed Mountain Lion System on the Recovery Partition? And tries to download the rest at installation? Then it's not legal, it checks ur system config and installs hence the prohibition. Pay $20 dollars and get it from the App Store. That would be legal! – aliasgar Aug 24 '12 at 22:50

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