If I'm correct, even if you follow Thinkdiff
's recommendations, you still won't see the disk. Seeing it once and then not seeing it again is the hallmark of a dead disk. If you can't find it under Disk Utility, it's dead.
What model computer do you have? How old is it? Presuming it's more than a year old and doesn't use flash storage, like the mac book air, your best bet to fix the dead drive is to buy a new drive from amazon, install it, then choose it as the drive to restore to (following Thinkdiff
's instructions for formatting the disk first)
The Momentus XT
drives have an SSD cache in them, similar to (but different from) how the FusionDrive in the new iMacs work. It's not as fast as an SSD, but it will be leaps faster than the hard drive you have in your mac now.--
I use Linux, OS X, iOS and Windows. Let the OS wars die.