rsync question

[Patton Echols]

I have an interesting issue with rsync

I have thumb drives I use to backup and transfer document related to 
various subjects.  For example, for project "foo" I have a directory in 
my home group named "foo" and one on a thumb drive named "foo"

To transfer from the thumb drive is accomplished with:
$ rsync -av /media/disk/foo/ ~/foo

This works fine, no errors. 

Transfer the other direction is the reverse command:

patton at Mycroft:~$ rsync -av ~/foo/ /media/disk/foo

which returns in part the following:
". . .
rsync: chgrp "/media/disk/foo/Membership" failed: Operation not 
permitted (1)
rsync: chgrp "/media/disk/foo/Officials" failed: Operation not permitted (1)
rsync: chgrp "/media/disk/foo/strategic-plan" failed: Operation not 
permitted (1)

sent 10092 bytes  received 1982 bytes  24148.00 bytes/sec
total size is 82023362  speedup is 6793.39
rsync error: some files could not be transferred (code 23) at 
main.c(977) [sender=2.6.9]
"


That is an excerpt, the same error message reports for every file and 
directory.  However,  a simple test shows that the transfer works 
correctly in both directions.  Any thoughts on how this could  be fixed?

fwiw, this is rsync version 2.6.9 on Hardy.

Thanks,
Patton