Install Mac OS on old Mac Mini not working from USB nor from internet A friend of mine gave me an old mac mini, the white one, i guess from 2007 or something. It has Mac OS 10.x installed (actually it has 3 versions installed for whatever reason).
The problem is that he has no idea what his user name and passwords are.
So I would like to re-install a fresh OS and setup the mac mini.
It is not the first time, that I do this, so I do press for sure the right keys while starting up.
I tried booting the system holding down the apple key + r without success. The system will boot into the primary installation.
With Command + Apple + r it is the same.
I can reset VRAM using Command Command + Apple + r + p and I hear the chime sound.
Pressing any other combinations doesn't let me hear any sounds.
So I wondered and created a bootable stick with MAC OS Sierra on it. Just to make sure my old mac mini can handle it. 
While booting up and pressing command I can choose between the installed Versions of MAC OS or install Sierry from USB.
When I choose to install from USB the Mac Mini will simply boot into the installed Mac OS.

I remember that on an old windows PC I wasn't able to install from USB even though he recognized the USB Stick. I had to do that from DVD / CD. 
What do you think, do I have to use a DVD/CD to finally install any OS?

Since I don't don't have the option to burn a disc here I wasn't able to try out yet. Maybe there is another solution?
First step is to erase the non volatile ram (used to be called PRAM) by holding Command-Option-P-R and listen for two boot chimes in a row then release the keys and try the install.

For an erase install, you need to boot to the installer and then choose Disk Utility from the Utilities menu and literally erase the drive. I would physically disconnect my backup if any before doing the erase. At that point, NVRAM or not - the system can’t boot from the erased drive and will make your life easier to select the correct installer and get a clean new OS.