My Apple G5 and WD Passport 500Gb don't mount

You need to determine whether the problem is at the disk interface or at the computer interface. Will the drive mount on ANOTHER machine using FireWire? Will your computer mount ANOTHER device using FireWire? This will tell you which machine is at fault. It's not unheard of for the FireWire channels in a Mac to just go bad, and (unless it is a Mac Pro) it typically requires a logic board replacement to address. For a G5, this would not be cost-effective. On the other hand, as I say, it could just be a bad interface in the drive itself. It's up to you to determine which it is.