Q: MBP won't start up with external hard drive

Because I should have done something else today.

1. I have a 2007 MacBook Pro with a failing hard drive that won't verify the volume. I have been nursing it along by starting it up with a Lacie Porsche external USB drive until the new MBPs are released this year.

2. I wanted to unlock my iPhone which requires backing it up, erasing it and restoring.

3. I used iTunes on the Lacie to back up the iPhone to the Lacie drive and erased it. When I went to restore it, iTunes refused to do so because it's too old.

4. To update iTunes I needed to upgrade Snow Leopard to Mountain Lion. I downloaded Mountain Lion and installed it to the Lacie.

5. Everything was going fine until the MacBook Pro restarted. It hung on the start up - saying it was installing (gray screen, little bar with a quarter of the meter bar darked out)  and stating another 30 minutes was needed. Two hours later, the bar meter was in the same place and still saying 30 minutes. I shut the MBP down and restarted. An hour later it's still hung in the same spot. Being an idiot, I restarted again.

6. Now the MBP refuses to start from the Lacie, it's own hard drive or the recovery thingy that pops up when I hold the Option key during start up. When I chose the Lacie or recovery thingy, it starts up but then shuts down in 20 seconds.

7. I can't use command R to get to recovery mode because that doesn't work on a 2007 MBP. The internal super drive is dead so I can't start it with the Snow Leopard disc or a third party repair disc. The MBP refuses to recognize the external super drive is hooked up to it.

8. I did once get it to start up with choosing the recovery thing as the start up and got to utility thingy. The Lacie drive is fine. I figured the issue was the MBP was confused and trying to start up with its dying hard drive. I chose start up so I could choose the Lacie and it would finish installing Mountain Lion. Only when I clicked and the box that popped up that should have shown me the start up disks - it was an empty box.

9. The MBP won't start in safe mode.

10. I went to single mode. The fsck commands can't verify the internal hard drive - so it can't repair it long enough to start it up so I can get into the computer, remove Mountain Lion from the Lacie and reinstall Snow Leopard.

11. I know one can transfer files in single mode and I presume a back up from the MBP's hard drive to the Lacie. Can I replace Mountain Lion on the Lacie with the Snow Leopard on the MBP in single mode?

I'm still stuck trying to figure out the Lacie's name that single mode will recognize. I know the MBP is disk0s2.

And getting the correct command that just transfers the stuff on the MBP (Snow Leopard) and not duplicate files is a bit challenging too. I think 'u' is used for just updating.

Is this possible?