If it isn't this [http://wdc.custhelp.com/app/answers/detail/a_id/8575/p/228,195,312/c/130/session/L3RpbWUvMTM0NDM4MDA3MS9zaWQvU2FSR3Q3M2w%3D|issue] ''you have either a wonky external case, or HD''. Often,''' as a lightbulb may "flicker", a failing HD may appear to work''' for a bit, then go bad. ''Should you get it to mount, backup all essential files.'' If you need to backup files, DiskWarrior, or Data Rescue 3 are two applications that I would recommend. If you have no essential files on it you can try a repartition... but generally once something like this happens you can't really trust that device for anything really important.
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