The program does not have the required permissions to erase the unused space on disk

I am trying to use Heidi Eraser (I guess it's just called Eraser these days) to wipe unused disk space on a drive. No matter what I do, I get the following error message:

The program does not have the required permissions to erase the unused space on disk. Run the program as an administrator and retry the operation.

I ran Eraser as administrator and I still get the error. I’ve tried taking ownership of the drive and it still doesn't work. What could be the issue?

Someone on the Eraser forums pointed out that you need to close the running instance of Eraser in the system tray and then rerun it as administrator. This worked for me.