Disconnecting the USB HDD from the PC.
Powering on but USB HDD is not detected.
Then what you have is a faulty power supply. This is especially common with enclosures or docks that come with power bricks. After long term of service the cheap capacitors in the power brick blow, just like mine.
Unfortunately, I was unable to salvage/scavenge the right capacitor as it was a 16v 640 or 670uf so I used a lower capacitor of 430uf and the dock works like a charm now. Below, are the replacement caps soldered on.
As you can see, when your dock or enclosure is acting out of whack it may not be the HDD or device itself but just only the power brick. And if you have some soldering skills and an old motherboard around you can immediately repair that sucker and give it a second life. Not to mention your wallet.