... Surely a powered hub is a different beast to an externally powered HDD designed to work over USB?
Or are we saying there is a design fault in the Pi here? ...
Yes they're different, but he's taking the power for the HDD from his laptop via a Y cable that's connected to the Pi on the other leg.
So it ain't the HDD that's supplying the power when it shouldn't, it's the Y cable that's supplying power from the laptop back up the Pi's input - cos it can't help it, being just a bit of wire!
Solution 1: mangle the Y cable so it doesn't send power down the data leg
Solution 2: remove F1 or F2 (whichever is relevent) on the Pi
Of note: If the proper power route for the Pi is another PSU - not the same laptop - the two PSUs will be fighting. Probably, F1/2 will shortly die, removing the problem