I have this same problem sometimes. I have several USB hard drives & flash drives. Whenever I have this problem, I simply choose to restart my PC. As soon as it all goes dark, just before the restart happens, I can & do safely remove whichever HD or Flash Drive that's hanging up. I got tired of tryig to find what was going on, & restarting the PC is the best solution for me. it is annoyng I know, but it works quite well for me, & takes just a couple of min. I only have this problem at times. It does not matter which one does it, I can use it just a few min later with no problem.