Erase and Install Snow Leopard
1. Boot from your Snow LeopardInstaller Disc. 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.
2. After DU loads select your 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. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed.
4. Select the volume you just created (this is the sub-entry under the drive entry) from the left side list. Click on the Erase tab in the DU main window.
5. Set the format type to Mac OS Extended (Journaled.) Click on the Security button, check the button for Zero Data and click on OK to return to the Erase window.
6. Click on the Erase button. The format process can take up to several hours depending upon the drive size.
7. When formatting is complete quit DU and return to the installer. Install Snow Leopard.
8. Download and install Mac OS X 10.6.8 Update Combo v1.1
Also not sure how erasing and installing Snow Leopard would help, as I'm using SuperDuper! to clone to this USB drive from my main drive, with all it's critical 3rd party software installs and authorizations, and the cloning process would just erase everything including any Snow Leopard previously installed on this USB clone.....
You had to erase the drive in order to use it on a Mac (it comes formatted in NTFS for Windows). Did you partition it, using a GUID Partition Table (under Options). It needs that to boot on an Intel-based Mac, which the Mac Pro is.