After mounting an exrenal hdd that is formatted as hfs journalled( with help of a post on this website). Can a dmg file downloaded from the net be installed to install mac osx on the external usb drive. My MBP only responds only in single user mode or the windows partition. It has mountain lion installed as an upgrade over snow leopard. Help will be appreciated.
You should use single user mode to delete the Mac partition (see the fdisk command).
Then, you should be able to download the installer over wifi (Hold down Cmd-R upon startup).