Info on preboot mode for Garmin devices is here: [Only registered and activated users can see links. ] and this is what it says for Edge:
Pre-Boot for navigators Edge:
before connecting the Edge USB press and hold (keep it pressed) Lap/Reset button
Your Explore is the exact same hardware as 1000 so preboot method should be the same, but i can't confirm without an actual device. So is that what you did? Holding the button [with unit fully OFF]? Then connect to PC which should power it on but put it into preboot.