Without external power, and only relying on the power from the USB, this is a typical and expected message for an external hard disk. But when powered from the wall it should work. So if it doesn't, either something is wrong with the power adapter (get your voltmeter and check if it still works, replace if necessary) or with the power-related electronics in the case (put the drive in another case then).
If you still have warranty, a simple visit to the shop should suffice. But it makes sense to backup the full contents of the drive, because they might decide to exchange it for a new one. This, by the way, is just an example of why it ALWAYS makes sense to have a backup of data on an external drive.
Kees
My Western Digital External Hard dive (Book) isn't being recognized by my Dell Latitude d820 laptop any more.In addition I am getting a Power Surge on Hub Port message saying the device has exceeded the power limits of the hub port. Then it take me to a window where I have to reset the port. But it doesn't fix anything. When I plug the device back in it doesn't show up anymore. It has it's own power source. I also have tried plugging it into the wall but that didn't help either. I have not had any surge problems. The external drive will work on my desktop. It will not work on any of the 5 USB ports. Sometimes I can shut down and restart. Occasionally the drive will show up but after about 3 minutes it will go out and I get the surge message. I have read all the other forums about similar problems and tried a few of the "fixes" but nothing is just like this problem and didn't work. I use this drive for my photos and need to access it using Lightroom. I really like using the laptop for this and hate to move it to my desktop. Any ideas? Please help me solve this problem.

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