tar question

[Ralph Alvy <ralvy@warpmail.net>]

I'm trying to create a compressed backup of my home directory in the
following location:

        /mnt/back

When I try it uncompressed with

        tar cvf /mnt/back/20031130 ~

it succeeds.

But when I try to do it compressed with 

        tar cvfz /mnt/back/20031130 ~

I get the following error message after it traverses the entire home
directory:

        tar: Error exit delayed from previous errors

What do I not understand about the tar command?