I'm talking about the CMD38 low-level command sent to the card by the Arduino. It has nothing to do with the file system. This is what cameras send when they do a low-level format. Windows and Mac are not able to send the command because the SD card reader (built-in or external USB) isn't coded to send the command. This is where an Arduino does a superior job. Instead of overwriting the entire card with zeros or FFs, it just erases the flash memory in a few seconds.