E: Read error - read (21 Is a directory) What is this

[Guy Durand <guy@millburncorp.com>]

I am having a problem with a friend's setup.  He suddenly cannot upgrade 
his system and gets this error whenever he attempts to do so.
I have done some searches but only found two references in spanish.

apt-get upgrade
Reading Package Lists... Done
E: Read error - read (21 Is a directory)

is what I get on the screen.