Very few of the PPCs ( your g5 ) will recognize an external usb dvd drive. You need an external firewire dvd drive.
Try holding down the option key then power on. This brings up the startup manager. Click on your hd. Click on right arrow key.
Sometimes if volumes don't appear in Startup Manager (what you get when you hold down the Option key at startup), you need to reset the Mac's PRAM, NVRAM, and Open Firmware. Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime. Then, before the screen lights up, hold down Command-Option-O-F until the Open Firmware screen appears. Then enter these lines, pressing Return after each one:
reset-nvram
set-defaults
reset-all
"The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."
http://support.apple.com/kb/TS1812?viewlocale=en_US
How to eject a cd from the internal cd drive:
eject cd
List of devices:
devalias
List of variables:
printenv
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If you have a second Mac computer, you may want to try to copy over as many files you can. One way is to use target disk mode.
http://support.apple.com/kb/ht1661
http://docs.info.apple.com/article.html?path=Mac/10.6/en/8443.html
"Installing OS X on DVD-Challenged Macs Using FireWire Target Disk Mode"
http://lowendmac.com/misc/06/0710.html
Here is a picture of a firewire port:
http://upload.wikimedia.org/wikipedia/commons/thumb/b/b6/IEEE_1394_Firewire_PCI_ Expansion_Card_Digon3.jpg/800px-IEEE_1394_Firewire_PCI_Expansion_Card_Digon3.jpg
