Reformat External Hard Drive with Operating System

Hi,

I have a hard drive I recovered from my old Dell laptop when the motherboard gave out. Originally, I just put it in an external hard case and have been using it as external storage. Now it is getting filled up, so I would like to reformat it to remove the OS partition (Vista) to give me access to the entire drive. I have removed and baked up all my files, but cannot find a way to repartition the drive to remove the OS volume. Disk manager will not let me access that volume to delete it - nor will diskpart. In fact, only the non-OS volume even shows up with a drive letter in diskpart.

I would really appreciate some help. I have searched the web but have only been able to find info on how to reformat the main hard drive...and this is definitely NOT what I want to do.

Thanks