I had this same problem and concluded that the drive had crashed or data was corrupted. However, it had been warm and the drive had been plugged in for a while. I tried it in a Windows PC and then later again in my Mac, but still didn't work.
Finally, I put it in the refrigerator and then plugged it in to another Mac and I was able to read it and extract the files I wanted.
Not sure if cooling it down was the answer, but this has also worked for me in the past with other drives.