OP wrote above:
[[ Hmm I'm pretty sure it's a late 2009 Intel on the Mac mini, and no idea about how to go about what you said to do for the G5 at all...]]
Well, then, perhaps it's time to start learning....
I believe Apple used SATA hard drives in the PowerMac g5.
You'll have to open the case and take the drive out.
This shouldn't be very hard, I would check "ifixit.com" for an illustrated guide.
Once you have the drive "in-hand", you need a way to connect it to the Mini.
The easiest and fastest way to connect a "bare" drive to a Mac is to use a "USB/SATA docking station".
To see what these are, go to amazon.com and enter "usb sata dock" into the search box.
Many choices will pop up, some as low as $20.
Just plug the drive into the dock and plug the dock into the Mac Mini.
Turn everything on, the drive should spin up.
It may or may not "mount" on the desktop.
Now you can open "Disk Utility" and re-initialize it if you wish.
At that point, it will be time to put a new OS onto the drive....