I'm a newbie and I had exactly the same problem. In the Forums I found two fixes, though there may be more. I used both; the problem has been solved and the empty space is happily being erased right now. Though I don't know if it was necessary to use both. I'm on Windows 8.1 and running Eraser Build 6.0.10 with the most recent updates (as at 17 Sept 2014) installed.
(1) In the Eraser FAQ, "Getting to know Eraser 6" is the second topic. It's a useful intro, and begins with the sentence "Read this even if you don’t want to read the manual". Halfway down is the following paragraph: "Running Eraser as Administrator - For some erasing tasks, and in particular when you erase free space on a drive, Windows Vista or Windows 7 may require that you run Eraser as Administrator (also known as 'running elevated'). To do this, first close the running instance of Eraser by right-clicking on the small Eraser icon in the System Tray at the right hand end of the Taskbar and selecting Exit Eraser. Then right-click on the Eraser program icon on the Desktop or the Start Menu and select Run as Administrator. Note that, when Eraser is running elevated, you cannot drag and drop on to the Eraser window from an Explorer window that is running normally; this is a Windows restriction."
(2) I ran a search with the keyword "permissions". The information you need is given in the answers to a question -- fourth down the list -- from alexandra18 titled "unused disc space".
Kudos to DavidHB for the 1st and Garrett01 and GregM for the 2nd.