Why thank you Windows

So I'm copying files onto an SD card to go into a digital photo frame and suddenly get this message:

Odd.  I check whether there's space on the card.  Loads.  I check that the file names aren't too long somehow.  Nope.

Turns out that the SD card is formatted as "FAT" -- the old, very basic DOS file system -- and there's a limit to the number of files you can place in the root directory.  Moving the new files to a new subdirectory solved the problem.  Great.

Now try explaining that to my Mum.  Particularly when she's received that wonderfully useful error message!