USB Target Disk Mode
Your MacBook supports USB Target Disk Mode when you use it with a compatible, full-featured (“super speed”) USB-C cable. Note that the USB-C Charge Cable (included with MacBook) does not support USB Target Disk Mode.
To enable Target Disk Mode, hold down the T button on your keyboard while starting your MacBook. Then connect the USB-C cable. Use a full-featured USB-C to USB-C cable to connect to another MacBook, or a full-featured USB-A to USB-C cable to connect to a Mac with standard USB-A ports.