Differential backup

[misiek]

Hello,

I have got three folders:
- /home/backup/2008-10-20 - place for differential backup
- /mnt/for_backup - folder with files for backup
- /home/backup/2008-10-01 - place where the last full backup is

My question: is below command prepared correctly to make differential 
backup?

rsync -avPbn --backup-dir=/home/backup/2008-10-20/ --exclude "System 
Volume Information" --exclude "RECYCLER" /mnt/do_backup 
/home/backup/2008-10-01

The reason I am asking is that after running above command, there are 
nothing in /home/backup/2008-10-20, although there should be, at least, 
some of my test files...

Cheers,
misiek