|There is a difference between the drive partition is full, and it doesn't have enough free space available for what you're trying to copy to it. |
An 8gb USB flash drive uses FAT32 software partitioning - the allocation units on it are probably larger than 4kb.
The movies on your hard drive are probably on a NTFS software partitioned partition, which always has 4kb allocation units in Win 2000 and above.
If you copy data (more than one file) that is approaching but less than 8gb in size on the hard drive, it may take up more than 8gb of drive space on the flash drive because the flash drive partition is using larger allocation units.
Also, no file can be larger than 4gb on a FAT32 partition. If any movie is larger than that, you can't copy it to a USB flash drive that is using FAT32 software partitioning.