3TB MyBook Essential no longer shows up in Windows Explorer, light flashes rapidly

WD External Drives External Drives for PC
2

When the lights are blinking, it shows there are activity in the HDD and slowing down means it is going into standby due to lack of activity or idle.

But blinking for 20-40 mins eventhough it is not being used is a little odd.

Have you try another power cable for the HDD?

Make sure it is the same voltage as the original though.