My daughter had an emac that needed to have OSX reinstalled on it. Some emacs came with a hitachi combo drive that would not boot an install disk. I had to do it with an external drive. It would boot to the original OSX disk that came with the Emac, but not any upgrades unless it was Emac specific. Other than that youcan install tiger and maybe 10.5 leopard if it is for ppc only
I just received an old iMac G4 with a burned out hard drive- after cracking it open and replacing the hard drive, I found myself in a tricky situation trying to get it to boot.
Here's the solution--
Convert your dmg file into an img file using the dmg2img program.
Insert your usb drive. Go into disk utility and select your usb drive (not the volume, the root drive). Click on the 'info' button.
There is a label called "Disk Identifier"- next to it will be something like disk1.
Open up a terminal window. You can do a direct data stream copy of the image file onto the usb drive by using a powerful program called Data Dump (invoked in bash via dd).
sudo dd if=/path/to/your/file.img of=/dev/targetdisk bs=4096
/dev describes any device attached to your machine. You should replace the targetdisk with the label of your usb drive found in Disk Utility. For the image file, you can drag the file into the terminal to give you the path of it quickly.
It may take a long time to finish and the program itself gives no feedback, but it works (could take over a few hours pending on the size of the image file).
dd is a low-level tool, if you target your main drive it WILL be botched/ruined.
Once dd has finished (the terminal prompt will return to normal), you need to boot your target device in open firmware (hold option-apple key-o-f).
Assuming there is only one usb drive plugged into the machine you're booting-- once you're in open firmware--
That should give you the installer.