Mac Mini 1TB HDD being 100% full - cant mount, cant boot
Create a bootable USB stick and boot the Mac off that. Make sure to use a MacOS version that is at least as recent as the Mac Mini’s date of manufacture, but not so new that it doesn’t support a 2007 Mac Mini. Apple’s instructions for creating a bootable USB drive are here. Then you can mount the hard drive (as long as you have the password, if it’s encrypted) and delete some files. Remember to empty the trash to actually get rid of them.