[CentOS] fstab, unusual behavior of "missing UUID"

I have the following as the last line of my /etc/fstab file on a 
computer running CentOS6.4..

UUID=3b550884-8d05-41a5-a205-17b6d7269dd1 /mnt ext3 
rw,suid,dev,exec,noauto,nouser,async  0  2

The UUID refers to an ext3 partition of a removable USB drive.

If the USB drive is not plugged into the computer the computer will not 
boot.  It seems that this is the incorrect behavior since "noauto" means 
there is nothing to mount.

I noticed this situation since an almost identical /etc/fstab on a 
Fedora 18 computer will boot without the USB drive being plugged in.

The reason this is important to me is that I want to plug my USB drive 
into a running computer and then mount it with "# mount /mnt" so I can 
do backups.

Thank you,
Joe Hesse