If you are copying files from your OS X partition to external drive or a Boot Camp partition and receive Finder Error Code -1309, it most likely means you are copying a file greater than 4 GB in size, which is the maximum a FAT32 formated drive can support. That's because the 32 in FAT32 means it uses a 32-bit unsigned integer for the number of bytes in a file, so 2^32-1 is 4 GB.
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