You must boot from another disk, not your internal drive, in order to erase the internal drive. For example, if you have a Snow Leopard DVD:
1. Boot from your OS X Installer 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.
Well, Kappy, followed your instructions, but after the hour-long startup disk install, everything on the DU page was still grayed out.clicking the 'partition' results in a box , blue and white, but all the volume scheme and volume information boxes are inoperable, as is the options button.
Oh, and couldn't see 'verified' anywhere.