External monitor not detected with MacBook Pro
From Apple: Can the HDMI port drive analog displays (VGA displays, for example) using adapters?


  No. The HDMI port will only drive digital connections (such as DVI)
  using the adapter that came with the Mac. The HDMI port cannot be used
  for analog displays such as VGA. You can purchase a Mini DisplayPort
  to VGA adapter separately to drive analog displays.


In addition to the actual digital signal, an HDMI port has a few optional pins for legacy analog support. Apple doesn't bother sending an analog signal to HDMI, so an HDMI to VGA adapter receives nothing.