Wonder if it's the problem of the dock or the harddisk ... seems like the dock
Let me start from the conclusion: it's always important to have backup gear in case of the running gear failed.
Here's the complete story that led me to make that conclusion: I have a series of USB 2.0 docks in my closet hooked up to a computer that I used as file server, and then share contents on my local network. Usually I will run files (like video files) directly from those shared folder with no performance issue.
But then since 2 days ago, I experienced problem access the W drive, which is connected to the front dock of the duo slot Cavalry USB 2.0 harddisk dock. Eventually I started to wonder if there's something wrong, either there's problem on the harddisk, or there's problem on the dock.
Luckily, I have a USB 3.0 duo slot harddisk dock, so I yanked out the W drive harddisk from the USB 2.0 dock and put it in the USB 3.0 dock slot. And things went back to normal.
With that, at least for now, I concluded that the harddisk itself has no problem, and strongly suspect that there's something wrong with the front slot of that USB 2.0 dock.