Dell Image Assist USB Creator tool seems creating problem for large USB drive

Dell Image Assist USB Creator tool seems creating problem for large USB drive. I created one bootable USB drive on a 80G USB. I cannot copy images (5G) to it even it reports it has lots space. The bootable USB disk only have very little space available when I plug in other computer.  I spend lots time to reformat and finally can use it. The 2nd time, i created an Lexar 32 boot disk, and the same thing happens. I have to use another large USB to input image and driver library when I restore image. A few day later, my Lexar bootable USB cannot be recognized in another other computer, of course cannot be used to restore image.  It seem damaged!