Also, have you tried restarting with the ext HDDs disconnected?
It kinda sounds to me like the HDDs are creating P&P errors that are then affecting your monitor. I say that since it renders 3 errors and you have the 3 drives. You don't have 3 monitors on a P&P config... So I'd lean toward it being an issues like that. Also - it's a really quick test!
1) Ensure PC is off.
2) Unplug
ALL 3 external HDDs.
3) Power PC back on.
If it starts then you can reconnect the HDDs and they usually work fine.
Anyway, it may not work, but many of my older Dells throw errors when booting with large USB storage attached. And it's possible that josteling the USB ports confused something.
If this doesn't work, then I'd go with the other suggestions. But this is a quick thing to try.