Internal Hard Drive - Disk Erase Fail - Can't Unmount Disk

This may indicate a bad drive. Open Disk Utility and check in the status area to be sure the SMART status says "Verified."

I would do the following:

Clean Install of Snow Leopard

Be sure to make a backup first because the following procedure will erase

the drive and everything on it.

1. Boot the computer using the Snow Leopard Installer Disc or the Disc 1 that came

with your computer. Insert the disc into the optical drive and restart the computer.

After the chime press and hold down the "C" key. Release the key when you see

a small spinning gear appear below the dark gray Apple logo.

2. After the installer loads select your language and click on the Continue

button. When the menu bar appears select Disk Utility from the Utilities menu.

After DU loads select the hard drive entry from the left side list (mfgr.'s ID and drive

size.) Click on the Partition tab in the DU main window. Set the number of

partitions to one (1) from the Partitions drop down menu, click on Options button

and select GUID, click on OK, then set the format type to MacOS Extended

(Journaled, if supported), then click on the Apply button.

3. When the formatting has completed quit DU and return to the installer. Proceed

with the OS X installation and follow the directions included with the installer.

4. When the installation has completed your computer will Restart into the Setup

Assistant. Be sure you configure your initial admin account with the exact same

username and password that you used on your old drive. After you finish Setup

Assistant will complete the installation after which you will be running a fresh

install of OS X. You can now begin the update process by opening Software

Update and installing all recommended updates to bring your installation current.

Download and install Mac OS X 10.6.8 Update Combo v1.1.

You can't. If you have Snow Leopard on an external HDD, then all you can do is clone that system. That will not reset your computer to its factory state ready for sale. To do that you need the original installer discs that came with the computer. If you have lost them, then you can repurchase them from Apple if the computer isn't obsolete.

Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.

To contact product and tech support: Apple - Support - Contact Apple Support.

What you are trying to do is essentially illegal. What's more you are not delivering the computer in its original factory state. You are merely cloning an entire installation from one computer to another.

There are no other legal options. As I stated above cloning your system to a computer you will be selling is illegal. This is an Apple owned site. We can't help you commit an illegal piracy of Apple software. I hope you understand.