If you have a Fat or Fat32 formatted external usb drive like I do to more easily move files from your mac to pc and ever get the message that there is not enough disc space to copy a file even though you are sure there is make sure it's not over 4GB in size!
I just ran into this problem the other day and remembered that FAT filesystem only supports 4GB max!
Windows 7 isn't helpful though and just gives you out of diskspace error though it should tell you the file is too big for destination or something like that!
FAT volumes have a maximum size of 4GB and a file size limit of 2GB.
FAT32 file systems have a maximum volume size of 32GB(this is only because limitation with microsoft's format tools) with a file size limit of 4GB.
If you want to format a disk that is larger than 32GB using FAT, ironically, you have to use an OS other than Windows like MacOSX, which I used to format my 400GB external as FAT32.