G5 is totally defunct, I trying to take out the HDD for archive purpose.
But realize ... there is no way for me to take out the casing.
The PowerMac G5's hard drives are fairly straight forward to remove.
Make sure to unplug the machine from power and disconnect all cables from the back of it.
Lift up the silver latch on the back side of the machine, this unlocks the door on the side of the machine.
Remove the door from the side of the machine
The hard drives are located in the upper right corner of the machine and are held in place by plastic tabs that rotate up and down
Flip/rotate the plastic tab holding the hard drive in place
Disconnect the two SATA cables from the hard drive. This is kind of a tight fit but rocking the hard drive & cables will help to release them
Once the SATA cables are disconnected the hard drive should slide out. The bottom one slides straight out, the upper one (if installed) drops a little bit then slides out.
iFixit guide for removing PowerMac G5 hard drives - Good reference with pictures
Here is the G5 manual (late 2005): https://web.archive.org/web/20130514170019/http://manuals.info.apple.com/en/Power_Mac_G5_Late_2005.pdf
On page 45 it is explained how to open the casing and onward to how to change storage devices.
Hope that helps!