I just bought a 2011 Mac mini which has two hard drives inside. One of the hard drives had a Mac Recovery I was able to transfer my data to with Time Machine, the other one is called "Windows" so I'm assuming it's formatted differently.
Can I format the Windows drive to install a Mac system on it without opening the computer?
Edit: After reinstalling twice, I am not sure there is a second hard drive inside the Mac anymore. I updated the question here. I will edit further this question once I understand what is going on inside this machine.
diskutil list
to your question so we can see. As this is a 'new to you' Mac, you ought to start by wiping the entire drive & installing a new clean OS.