Just a guess at this point without more details, but....
Is it possible that your flash drive is formatted as a Fat32 volume? (4 gig file size limit).
You may have to reformat to NTFS or just CONVERT X: /FS:NTFS (where X: is the drive letter of your flash drive).
The first will erase the drive completely of course.
The second is generally non-destructive but I would still back up any critical data on it before I tried it.