29 October 2021
Use an oil-based felt-tip pen with a soft nib to write only on the label side.
Regarding how to distinguish between the label side and the recording side, the recording side is a plain mirror surface, and the label side is designed with capacity notation and manufacturer name. In the case of an inkjet printer compatible disc (printable) type, you can also write with a water-based felt-tip pen.
(Since both sides of the double-sided disc are on the recording side, do not write with a pen, etc.)