chgrp problem

[keith.briggs at bt.com]

I'm using a comand like 

rsync -av directory /media/usbdisk/

to backup to a usb device (on a Fedora 5 machine).   I'm getting 100s of messages like 

rsync: chgrp "/media/usbdisk/file" failed: Operation not permitted (1)

and then at the end 

rsync error: some files could not be transferred.

How do I fix this?   Why is rsync trying to change the group?   And is the message at the end correct? - it actually looks like all files were transferrred.

Thanks,
Keith