If you have a display problem, the likely culprits are:
4. Electrical Short
5. Display driver
By booting without the OS drive, barring any other problems, you should see a message like "No boot device found". Seeing this message would indicate that the GPU/chipset isn't the problem. If you haven't already, take a look at our troubleshooting checklist (link in my signature), and if you still have problems, start a new thread with the results of the checklist tests.