moving images to USB

I have several thumb drives and have used them to export tiff images from iphoto to the thumb drive before. I've deleted the images on them and wanted to do the same thing with additional images, but I get a message on each of them that there isn't sufficient space...even though they're empty. One is a 4gb drive and the other is 16gb, so size isn't the issue. Any ideas?

Plug in your thumbdrive, pen drive, USB stick whatever you want to call it back in and empty the trash can.

Well, that was easy. They're copying to the thumb drive now. Thanks...don't know why I neglected to do that!