Do you mean 'Dedicated' graphics card and not an external one ?
Remove the card and using the built in graphics, enter the bios and set the Graphics card to initialize first to PEG/PCIE. Set Secure boot to off/disabled, and set CSM or Compatibility Support Module to on.
Save settings & power off.
Install the 750Ti and boot back up
It's an external one.I did what u said but its showing black screen.