In 2011, I bought 2 units of WD My Book Essential 2TB external HDD to backup my working files.

Unit A is being used at least once a week for backup, while unit B is not being used regularly, probably just a handful of time 2-3 years ago.

Now that unit A is full, as I took the unit B from the cabinet and connected it, it doesn't seem to be RECOGNISED:

• I got the pop-up message saying the device is ready to use.
• It appears in My Computer, but without the capacity information. Double clicking it doesn't open it, just the green loading bar.
• It appears in Device Manager under "Disk drives", but under "Other devices", there is a "WD SES Device USB Device" with yellow exclamation mark.

So today I bring it to the computer shop, same problem. The technician said the HDD is dead and there is nothing can be done.

My first question: How possible is it that a HDD that's rarely been used will die for no reason?

Weird enough, after I reach home and try to connect the HDD again, this time I get the  scan the disk to fix problem message (can't remember the EXACT message), and after the scanning is completed, the HDD just works as normal!

My second question: How can I be sure whether the HDD is really healthy?

I'm hesitated to use it, worrying it may be dead again anytime. 

Anyway, the computer shop technician suggested that I should get a mobile HDD instead (which does not REQUIRE power input) since I'm only using it for backup purpose, and it's slightly cheaper (not much though).

My third question: Practically, what are the differences between the two types of external HDD (one needs external power and one doesn't) ?

Thanks in advance. My first question: How possible is it that a HDD that's rarely been used will die for no reason?

HDDs don't die for no reason, they die for a reason. You just don't know why this one died. An HDD can die at any time, even after being in storage since new. In fact in a long-stored HDD the spindle lubricant can pool at one end of the spindle and prevent proper spinup the first time it is powered up.

My second question: How can I be sure whether the HDD is really healthy?

You can run the maker's diagnostic tool, WD DATA Lifeguard Diagnostics, but you still can't be sure an HD won't fail. That's why we do backups.

My third question: Practically, what are the differences between the two types of external HDD (one needs external power and one doesn't) ?

An external drive without a power supply (wall wart or AC power) gets its power from the USB socket it is connected to. You need to be sure the USB socket can supply enough power. If it can't you can overheat or damage the computer system or hub and/or get flakey operation.  With an independently powered HDD, you don't have to worry about that.
Is the other WD plugged in when testing the 2nd one ? ? Quote from: Salmon Trout on March 25, 2016, 06:03:11 AM

HDDs don't die for no reason, they die for a reason. You just don't know why this one died. An HDD can die at any time, even after being unused and brough out of storage. In fact in a long-stored HDD the spindle lubricant can pool at one end of the spindle and prevent proper spinup the first time it is powered up.

An external drive without a power supply (wall wart or AC power) gets its power from the USB socket it is connected to. You need to be sure the USB socket can supply enough power. If it can't you can overheat or damage the computer system or hub and/or get flakey operation.  With an independently powered HDD, you don't have to worry about that.

Is the other WD plugged in when testing the 2nd one ? ?