Here's a photo of a 520. It's the "Hard Disk" port all the way on the right. https://upload.wikimedia.org/wikipedia/commons/c/c0/Rear_ports_of_original_Atari_5

Here's a photo of a 520. It's the "Hard Disk" port all the way on the right. https://upload.wikimedia.org/wikipedia/commons/c/c0/Rear_ports_of_original_Atari_520ST_%28crop_and_filter%29.jpg https://upload.wikimedia.org/wikipedia/commons/c/c0/Rear_por... It was possible to hand-make a replacement for Atari's external floppy drive (round DIN connector on the right) because Atari wanted too much money for theirs. Having two floppies meant that compiling went from several disk-swaps to no-swap happiness.