How to take out HDD (hard drive) from a G5
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!