True Image 10: How to wipe backups from ext HD

I have a 500 gb Seagate Free Agent HD that was used to store backups for Acronis True Image 10. The HD was partitioned creating a 350 gb secure zone. The laptop that it was used with no longer exists and I want to use the HD for other things. Connecting it to may Win 7 desktop shows me only a 150 gb drive. Apparently the secure zone created by Acronis is not visible and can't be formatted. I still have the Acronis 10 source disk but it is not installed on my PC and I am reluctant to do so. Is there a way to delete the partition on the HD so I can use the entire thing?

I was advised to delete all the partitions on that drive through the disk manager of my Win 7 64-bit OS but only the 150 gb partition and a 80 gb primary partiton are present along with my own backup drive and my system drive. The majority of the E: drive (Seagate Free Agent) is not accounted for.