False "Disk is Full" Message

I have a new MyBook Essential 2TB hard drive. I use it primarily to backup files of JPGs. There is plenty of room on the drive, over 1.5TB. However, when I open a JPG in PhotoShop, make some changes, and try to save the modified file, I get the message “Disk is Full”.  Nonsense! Why can’t I save files to the drive. It is not Read Only.  Help!

Have you verified with the problem also happens with any other type of programs?