Scrubbing disk and reinstalling OS

If I back up my MacBook Pro OS to an external drive, can I reformat the disk and then reinstall Mt. Lion from the external drive?
Thanks in advance for the help.

How you do that depends upon what you use to do the backup. if you make a "clone" of your internal drive then yes that works. Internet connection to re-download Mountain Lion. If you are using a Time Machine backup you would need to boot from the recovery partition and restore files from your backup. That might also mean re-downloading Mountain Lion if you did not include iy in the Time Machine backup.

Here's what I would do. It leaves you with the most flexible option in the event of a drive failure.

1, Download and install Carbon Copy Cloner.
2. Clone your internal drive to the external drive,
3. Reboot your Mac holding down the option key. When you get to select the drive to boot from select the external drive.
4. If everything is running properly from the external drive go to the utilities folder on the external drive and run Disk Utility to format the internal drive.
5. Now run Carbon Copy Cloner again and clone things from the external drive back to the internal drive.

A follow-up question

Thanks. That's very helpful. I've also read that In Disk Utility, I could select the disk or volume in the list with the free space I want to erase.

Then click Erase and click the Erase Free Space button.

Would this get my disk cleaned off except for the OS and would it be a simpler solution?

Thanks again.

Not if you want to start with a clean installation of the OS. Are you trying to start with a clean installation of OS X?

If you are doing a clean install because of a problem let us know the type of problem. A clean install may not be needed.

Actually, what I'm trying to do is clean most stuff off the Mac except for the OS so I can sell it. I would like to keep; some of the apps for the eventual owner but would forgo that if I could get an easy solution.

Does this help?

Clone as suggested using SuperDuper or Carbon CopyCloner, remove the apps you wish to keep and then set up a new account and delete your existing account.

Actually, let me stop you right there. I understand exactly why you'd want to give them a copy of the OS on this machine you're selling them, but in fact that's a mistake (and a violation of the license).

If you give them a copy of Mountain Lion, it will be your copy, tied to your Apple ID; and it will require your password every time they need to upgrade it. I think you can see why this is a bad idea.

Wow! I never thought of that. Thanks for bringing this to my attention. So what would I have to do to include an OS? I see Mac products on Amazon all the time that have OSes.

Well depending on the precise model which has not been stated, Snow Leopard OS X.6 DVD is available from Apple Online for $20 including postage. Snow Leopard will run on up to a late 2010 model and maybe early 2011.

Actually, Snow Leopard will run on a Mid 2011 iMac. Later that year, the 2011 iMacs were shipped with Lion.