24' iMac HD problems

Can't erase internal hard drive after booting from external drive. Have attempted to erase using Disk Utilities but all options greyed out.

Hi padowns

Are you trying to restore a back up? Is it a Time-Machine, Carbon Copy Cloner or SuperDuper back up?

Hi Spawn_Dooley

The background to this is: I am trying to erase the HD so that I can load Snow Leopard on to the iMac using target mode. Snow Leopard will not load via the installation disk to the iMac because (it says) the iMac HD is not a bootable volume, even though it clearly is in practice. I tried the target mode before without erasing the HD but that was rejected because the system folder could not be modified, I renamed the system folder but that did not work either.