My Mac would boot to blue screen and stop there, I didn't know this and started it up then left for work. 10 hrs later when I returned it was still running grey screen only so hot it scorched the top of the screen. After shutdown and cooldown I tried agaain, same grey screen. I unplugged my WD Essential, and bang I got my aapple symbol.
I have reformatted to my Mac, and still must leave it unplugged. Why doesn't Apple make an external hard drive for this?
IIRC the WD Essential are a USB powered drives, is that correct?
I have no real experience with any of those type drives, USB powered, except for one a friend bought and could not get it to work on his Dell notebook because the Dell was no putting out enough power on the USB port to power it up properly. He had to get one of those Y cables to connect it to 2 separate USB port to full power it.
If it is one of those USB powered drive then your Mac is hanging trying to feed it enough power.
If it isn't one of those USB powered drives, it has its own AC adapter, then you may have a problem with your Mac and Apple should check it out.