TomGeorge:
As a rectifying diode yes, but as Back-EMF diode they need to conduct on the reverse polarity of the coil Back-EMF, and not conduct when the coil is being powered.
Tom...
Right. You connect in reverse across the inductor because when supply current stops the inductor output sources a current surge that the diode loops back into the inductor but it can only do that when the supply power is OFF, since supply power ON keeps the diode output blocked. The diode still only conducts charge in one direction.
That's weird though in the - you connect to will be + and the + become -........but only when switched off.