I'd install rEFInd in my Mac Mini 2,1. It has a OS X Lion bootable partition and a Windows 10 install ntfs partition. I tried to boot from the ntfs partition but the Windows Installer won't run, only a black screen with a DOS cursor blinking. Now I can't make the rEFInd boot menu show up again. I turn on with "alt" key or "c" key pressed but Mac Mini ignores and (appears to) try to boot from ntfs partition again...
Any idea before I have to open the Mac Mini physically?