Image file disappers from thumb drive

I am backup up a Win7 system onto a 64gb thumb drive once per week. Now and then though I need to use that thumb drive to transfer files to winxp machines. The funny thing is the win7 system backup disappears. I check the original system and the backup has been completed successfully. Right after the backup I check that the .tib file is on the thumb drive - it is (38gb). Then a few days later, it's no longer there. What could be causing this?

Thanks.