I have a 1TB USB drive connected to my Airport, and sometimes when I try copying large files (multiple GB) files to it I get the following error message:
Sorry, the operation could not be completed because an unexpected error occurred. (Error code 0)
First I thought maybe it was a problem with using my AirPort, so I tried connecting it directly to my Mac, same error.
It turns out the problem is that the USB hard drive was formatted using FAT32 (this is how they often come from the manufacturer, I never reformatted it), which only allows files of max size of around 4gb, when you try to copy anything larger you get this unexpected error.
The solution to this problem is: