Is there a way I can get my G5 and my macbook pro to share an external hard drive? Seems when I try to do this, it is a race between the machines. One of them gets it and the other one won't finish booting up.
It sounds like you have the two machines (CPUs) and the HD hooked up to a USB hub.
If so - this is WAY bad. Stop now.
FYI - You can connect as many USB devices (printers, HD, mice, keyboards, scanners - etc) to one CPU as you want - but you can never attach two CPU's to the same USB hub.
The proper way to share the HD is to connect it to one of the two machines (pick the one that stays on the most) and "share" the HD from that one. Then, "map" the remote drive to the second machine.
This will allow you to access the files from either machine.
But - you will still have problems with "file locking" - which means it you are editing one file at the same time on both machines, the last machine to "save" the file wins, and all updates that were created by the other machine are lost.