Monitor Startup Issues
The Raspberry Pi 3 B has an HDMI connector for monitor and audio. If you wish to use a VGA or composite video , an adapter cable is required. Not having a spare HDMI monitor, and having old but perfectly good VGA monitors I bought a Unitek Y-6333 adapter cable. This did not work until I realised that external power must be brought to the Y-6333 before the pi boots. Once the pi has booted the external power is no longer needed. It seems that the pi does not detect the hdmi device in time. There may be some way in software which I haven't found to enable the HDMI display without needing to do this.