The flash drive may be of a format that can not be accessed through the mac.
I would recommend you connect the flash drive and right click > get info and check what format it is in. Also, could you provide us with more details of what mac version you have?
The passports are generally partitioned with the FAT or FAT32 partition scheme, and you're probably trying to put a 2GB file onto the drive. If you want this flash drive just to be used with Macs, I would reformat the drive with MacOS journaled in Disk Utility, so you can do whatever you want with it. Now if you want this drive to be available on Macs and PCs, and want to transfer files 2GB and larger, then you might want to look into getting a NTFS driver for OSX so you can read/write any Windows partitioning schemes on your Mac. The one i use is Paragon NTFS, but i know there's another out there called MacFUSE, though i have no experiences with that one.