Q: can't recognize my hard drive to boot

Try this:

Install or Reinstall Lion/Mountain Lion from Scratch

Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Erase the hard drive:

  1. Select Disk Utility from the main menu and click on the Continue button.

  2. After DU loads select your startup volume (usually Macintosh HD) from the

      left side list. Click on the Erase tab in the DU main window.

  3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on

        the Security button and set the Zero Data option to one-pass. Click on

      the Erase button and wait until the process has completed.

  4. Quit DU and return to the main menu.

Reinstall Lion/Mountain Lion: Select Reinstall Lion/Mountain Lion and click on the Install button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible

            because it is three times faster than wireless.

Review these two articles for directions on reinstalling your OS:

https://discussions.apple.com/docs/DOC-5711

https://discussions.apple.com/docs/DOC-4571

Ciao.

if the HD doesn't show up as option to install to you may have filevault or firmware password enabled. if so you will need to enter that to unlock the HD before being able to erase and install the OS on it. following articles address these 2 items:

http://support.apple.com/kb/HT4790

and

http://support.apple.com/kb/TS3554

good luck!

Its very certian that your HD has one foot in the grave

Boot Using OPTION+R keys:

  1. Restart the computer.

  2. Immediately after the chime press and hold down the

      "OPTION" and the "R" keys together.

  3. Release the keys when the Utilities Menu appears.

Select Disk Utility from the main menu and click on the Continue button.

You will need to select the sub-entry, disk0s2. Click on the Mount icon in Disk Utility's toolbar. See if this will remount your "lost" volume. If nothing happens, then you need to re-format the volume:

1. Select the volume, disk0s2, from the left side list. Click on the Erase tab in the DU main window.

2. Set the format type to Mac OS Extended (Journaled.) Change the name of the volume to "Macintosh HD." Click on the Erase button and wait until the process has completed.

If this works OK, then continue by quitting DU, selecting Reinstall OS X from the main menu, then click on the Install button.

you can do backup but only if the mac will bootup to use time machine.

other options are if you own another mac can try target disk mode to retrieve your data on the mac. this article tells how to use target disk mode:

http://support.apple.com/kb/PH10725

Good thing he fixed the drive before having to replace it.

Should have listened to the first advice before clicking the Solved button so soon.

I concede on that ,...however nobody here mentioned partitioning the HD, ....unless you meant partitioning the resident internal HD when you mentioned the "backup to the second internal drive".  

He just gave credit to the wrong guy (me)