had this to replace a Samsung external drive that had poor access times 10 second lags etc.
Unfortunately this started to display the same fault so I tested my USB ports but they were fine, I even bought a USB 3.0 PCIe card and tried that, speeds were great for a while then access times dropped again, this eventually culminated in me getting a message on-screen asking if I wanted to format the drive... WHAT!!!! over one gig of data gone... Hmmm I don't think so.
I decided that maybe the USB connection was at fault, the circuit board maybe, so, I removed the HDD from the drive and installed it as an internal drive.
problem sorted, no lag, no lost data. I then got to thinking that maybe these external boxes are underpowered on the system, I personally have a CIT 750watt modular PSU and its powered everything i've had, I'll be upgrading to a BeQuiet 900watt soon and may try putting the HDD back in the box.
I've seen systems that have cheap PSU's or PSU's that really don't give enough juice displaying these characteristics. just a thought don't have time to test the theory.
anyway now its running well, the drive is a green drive so its not going to be super fast but I got it for size not speed. I give it three because it now works but I shouldn't have had to do all the above.