Spelling differences like disk and disc can be frustrating. They make us writers ask: Why!?!?
In this case, the etymology takes us back to a word that refers to something flat and round. In Latin, the word was discus (for quoit, disk, dish). Meanwhile, the Greeks spelled it diskos (coming from the verb dikein, meaning “to throw”). Of course, that diskos that Greek athletes hurled in the ancient games is today the discus in the modern quadrennial games.
Enough history, we know. You just want to know: When do I use disk vs. disc?
Honestly, it’s all over the place throughout history (see our pals at Merriam-Webster for more).
We opt for Associated Press style here: Use disk for computer-related references like floppy disk and for medical references, such as herniated disk. Use disc for optical and laser-based devices like a Blu-ray Disc or CD and for disc brake.