Strange situation in making backup on an external disk
I'm guessing that volume's format is FAT32 and you're trying to copy file bigger than 4GB. That's FAT32 limit. Check this for a reference.

As the first point of investigation, check what's the format of your partition. If it's FAT then that's the reason. Change it to exFAT, NTFS or native OSX.

So available free space has nothing to do here and can be misleading.