Have you tried it with other computers? Is it only when you plug it into your laptop that you have the issue?
I have three computers I use semi-regularly
3. TV Server
What I usually do is copy files to the portable and then plug it into my laptop in the wee hours at work. When you are at work, you don't get that many options.
Well, the first thing I'd do is find another computer to plug the HDD into and see whether the files 'disappear.'
I renamed a number of video files to be able to better organize them.
I also made a copy of a new blu ray I just bought.
Copy them to the external via the USB 2.0 port
As soon as copied, I looked at them on the TV server computer and laptop.
They were all there.
It does not happen "on command" so going to have to wait and see. If it is something weird with the 3.0 port, probably see if I don't have any issues after a few weeks, maybe a couple of months.
The old Samsung did definitely fail its drive test and have to wonder why the WD 2 TB never had any issues with the USB 3.0 port.