Boot from your bootable backup drive - restart the computer and after the chime press and hold down the OPTION key until the boot manager appears. Select the icon for your external drive then click on the downward pointing arrow button.
After the computer starts up do the following:
Drive Preparation
1. Open Disk Utility in your Utilities folder.
2. After DU loads select your internal hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area. If it does not say "Verified" then the drive is failing or has failed and will need replacing. SMART info will not be reported on external drives. Otherwise, click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID then click on the OK button. Click on the Partition button and wait until the process has completed.
Leave DU open and do the following:
Clone using Restore Option of Disk Utility
- Open Disk Utility from the Utilities folder.
- Select the destination volume from the left side list.
- Click on the Restore tab in the DU main window.
- Check the box labeled Erase destination.
- Select the destination volume from the left side list and drag it to the Destination entry field.
- Select the source volume from the left side list and drag it to the Source entry field.
- Double-check you got it right, then click on the Restore button.
Destination means the internal drive. Source means the external drive.
This will restore your old system so you can continue using the computer.
An alternative to the above is that after formatting the internal drive restart from the Snow Leopard DVD and install Snow Leopard on the freshly formatted internal drive. You can then restore your Home folder and other applications through the Setup Assistant which will provide an option to restore your files from another Mac or hard drive.