WD Elements4TB AC Adaptor,is it needed? Works without!

This is my first experience with this product. Other external drives I have need the ac adapter to function. When I plug the adaptor in , the drive turns off! The adaptor has 12V output, when checking with voltmeter.Confused.
Update: This happened only the first time it was connected.Now is normal operation. AC power is now needed as you would expect. To recap:first time I connected usb cable the drive powered up and was seen in windows. This was without the AC adapter. Second time the adapter was needed to power on. Don’t understand, but that’s what happened.Is this normal operation?

It could have been a residual power issue. All WD My Books and non-portable WD Elements require a power adapter.