Just to make sure it’s not some software on my Macbook blocking stuff, I fired up a Live-Version of Ubuntu, set it up, same issue… 
There’s gotta be a different way of doing this…!
This is where I’m stuck. BTW: I just renamed the file to “Firmware.bin” just to make sure it’s not a bug with underscores etc. That being said, it’s not working with the original file name either
UPDATE I erased the HDD before repartioning it, like Fox_exe proposed. This command did the trick:
dd if=/dev/zero of=/dev/sda bs=1M count=10000
