iMac A1312

The hard drive on my 27" iMac A1312 (circa 2010) recently died, and I have fitted a new drive myself on the advice of Apple's Genius Bar. I work as a maintenance engineer and am used to taking things apart and putting them back together carefully!

I also bought a Snow Leopard 10.6 bootable DVD-ROM, which I have connected via a USB DVD-ROM drive as the built-in drive hasn't worked for a while.

I am trying to boot from the disc, holding the Option key while powering up. The external disc spins up straight away, then I get a white screen. After a few seconds the mouse pointer appears, followed by an icon for the DVD with the message "Mac OS X Install DVD".

When I click on the arrow underneath, the screen remains white, the DVD icon is replaced by the familiar Apple logo, then the DVD spins down to a stop and nothing further happens.

Possibly a side issue, but might be part of the problem - I don't hear the fan spin up at all, and the top of the iMac gets very warm after a few minutes. I have had the screen off again and double-checked the connection of all 4 cables that were disconnected to fit the new HDD.

As I have fast broadband, I am also trying Internet Recovery Mode, holding cmd-R. Same white screen to start with, but nothing else except the "?" folder icon, meaning it can't find a boot source, although the router tells me the iMac is connected. Cmd-option-R gives the same result. In both these cases, no mouse pointer appears, and the DVD doesn't spin up at all.

Any advice greatly appreciated. Many thanks.