In my experience, dragging files to the trash off a CD-RW or a DVD-RW or a USB key (and emptying trash) will delete the file from the list of contents, but won't actually clear the filespace on the device.
To clear the space, you have to go to Disk Utility and reformat the whole thing.
Was a bit suprised by this, the Apple Help isn't very clear on this issue.
At one stage my USB key apparently had no space left on it even tho it had no files on it whatsoever. Not very helpful to the newbie 🙁
xoxo ..RedTomato..