Thank you for the opportunity to help you with your question!
Magnetic disc is written in this way that its first prepared in to tracks and sectors. The read and write heads move on the surface magnetizing and demagnetizing. The magnetized parts will stand for a zero while the unmagnetized part stands for a 1.
check the images below
Blue ray technology does not work on magnetic discs. It works with optical media like DVDs and Cds
Please let me know if you need any clarification. I'm always happy to answer your questions.