This is not really a MBA problem, but probably either a USB or Mac problem, but could not post to the "Mac Hardware" . I have a brand new WD USB drive that is attached as follows to my 2011 MBA Ultra>>>ACD TB >>>USB hub >>> WD HD. I am trying to create a new back up drive, but it keeps disappearing from my finder and even disk utility. Occasionally, I will see an error message saying that it was not ejected properly. Sometimes, it even works. I have changed USB ports and that sometimes works and sometimes does not. I should say that I have also had this problem with an older USB drive which is why WD sent me this new one under warrantee. But not now, I am not sure if it is really a problem with the drive or the USB connections or something on my MBA. It is rather erratic, but seems not to work most of the time. Suggestions?