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I got a iMac from a friend and it originally had mountain lion. I was able to login and use it like normal for about 3-4 times. Then it won't log me at all.

I got apple and some other experts and was recommended to change the hard drive. I got another hard drive extacly the same size, same company etc.

I also bought snow leopard from apple store. I start the iMac and hold down the "option key" and I select the DVD installation (hard drive doesn't appear here) and when I select it then it goes to a white screen with grey apple logo and it nothing else happens from there onwards.

I can't access the iMac at all e.g. disk utility and so on.

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If you can't see the hard drive in the installation even though it's brand new, you have a hardware error somewhere else. Possibly the motherboard or the hdd > motherboard connection. – pzkpfw Feb 28 '13 at 20:22

First try wating a little longer for the DVD to load. If not success, try using an external USB DVD-RW drive and boot the installation DVD-ROM from there. Assuming the Mac has no other problem, you should be able to start the Installation. If you suspect RAM or other problem, hold D while booting to enter diagnostics mode and check the RAM for errors.

Since the hard disk is new, it is not yet bootable, that's why it doesn't appear as a selection when entering boot selection mode. After successful installation of MacOS it should appear as a choice.

I hope that helps.

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