"Couldn’t create temporary file." error when disk erasing free space

I was trying to wipe the free space of an external USB 3.0 disk in OS X (Yosemite) disk utility. However, I receive the following error:

The disk is formatted as GUID / Mac OS Extended (Journaled) and 1.1 of 2.0 TB are occupied.

1. Do you know what the cause / solution could be? Disk Repair returns no errors.


2. Do you know of a terminal command to wipe free space?

Thanks!