For completeness, you should provide the exact version of Guardian Edge you are using. Like you, I am unfamiliar with this product, but a quick Google shows that Symantec acquired this company in June 2010.
So if you go to the Security Community in Connect, and find the Endpoint Encryption forum, you should be able to get further information about Guardian Edge from the security specialists there.
If you already have a WinPE boot CD or USB stick that supports this model, you could boot to a WinPE command prompt, run DISKPART and try deleting and recreating the primary partition on the encrypted hard disk. If this works, then you can move straight to reimaging. If the hard disk cannot be accessed, then assuming the correct SATA drivers are loaded, you will need to decrypt first.