Overwrite MBR with custom message
First partition of the your disk. Located in \.\PHYSICALDRIVE0 and size of 512 byte. If it's overwrited, you won't able to turn on your PC. However, there are fix (bootrec /rebuildbcd && fixmbr) command that can revert back your mbr to default state.
Standard - Flat overwriting can be done with empty 512 length byte array which will cause this screen;
byte[] data = new byte[512];To write custom message to this screen, you'll need single 7-bit ASCII array which contains ASCII of the each char. You can convert your string to byte arr with;
byte[] bBytes = Encoding.ASCII.GetBytes("the custom msg to write");MBR overwrite may broke your system. Use at your own risk, recommended using in vm.