How to remove Windows partitions on external HDD?

Hi! I wonder what is the correct procedure to remove all partitions from an external HDD (500GB) that was previously a Windows boot HDD (Extracted from my old PC). Before the erase, there were 3 partitions:

1. Win7
2. System Reserve
3. Data

After backing up and erasing the drives, they're now empty. I'd like to remove all the partitions but it doesn't seem to allow me to delete by clicking "-" from the Volume on the 2nd top left on the Toolbars.

Any idea how do I do it?

Solution 1:

The key to do this is to click on the view icon and select show all devices.

Right now you can only see volumes. When you enable show all devices you will see the actual physical drive. Select it and erase the drive.