[ubuntu-uk] rsync chgrp problem - advice, please?

[luxxius]

I'd be grateful for a bit of advice about something that's probably 
utterly obvious to anyone but a complete noob like me.

I'm backing up 3000+ music files to an external usb drive by running

	sudo rsync -av /home/diana/music/ /media/USBdisc/Music

A lot of the material was already on the destination drive;  but some 
files have been newly created on the source only.

Everything seemed to work OK, apart from half a dozen errors of the 
kind I've put between the hash marks below (all these errors were either 
to do with files recently created on the source, or with the top-level 
folder on the destination drive):

######
rsync: chgrp "/media/USBdisc/Music/Podcasts/In Our Time/65. William of 
Ockham 31 May 07.mp3" failed: Operation not permitted (1)

rsync: chgrp "/media/USBdisc/Music/." failed: Operation not permitted (1)

sent 218669 bytes  received 24480 bytes  486298.00 bytes/sec
total size is 75452289335  speedup is 310312.97
rsync error: some files could not be transferred (code 23) at 
main.c(977) [sender=2.6.9]
######


I think (I hope!) this may mean that all the files were actually 
transferred,  but rsync could not synchronise some group permissions. 
Is that right?  Or were some files actually not transferred?

What might I do to make sure this works properly?

TIA
--
Diana