Just a possibility...I purchased an ACOMDATA external HD 3-4 months back. Installed and it worked perfectly the first time. After that it would not show up just as you described. I contacted ACOMDATA and they said that I need to turn it off when finished using it and on again when I need to access it. I can of course leave it on once I have it turned on as long as the computer itself is on and working. Since then I have had no problems. Still I find that have to turn it on each time is rather odd but I can live with that.