DBAN for Mac
DBAN for Mac
You might try using DBAN for Mac on your Mac Mini. Since you can’t boot from the disc with the 'c' key, ensure you have the correct installation media ready. If needed, follow the instructions carefully to complete the data erasure process.
Press cmd+r to enter recovery mode. Use disk utility to erase the drive and reinstall OS X.
Avoid using weak methods to erase data; consider robust tools such as DBAN for secure deletion.
You can find settings for actions such as 35 passes in the main menu or options section of the app. If you haven't noticed them before, check the help or settings area.
Under the security settings, even a 7-pass wipe seems excessive from my perspective, though it's probably safe.