Then it is most likely a bad HDD. Shut down the unit and remove drive 1 and see if the unit will boot up. If it doesn't, shut the unit down again, replace drive 1 and remove drive 2. Once again see if it boots up and if it doesn't then just continue this process until you find the bad HDD. Once you have found it you just need to replace it (if its within its warranty then call support for a replacement).