Backup entire system on usb disk

["Zacharie Elcor" <zelcor@xxxxxxxxx>]

Hi all, 

I made a fedora install on a bootable usb disk to have a system backup in case my laptop internal disk crashes.
This works ok but I'd like to be able to perform regular updates of the usb disk (including packages installed or updated by yum and personal files) so that the usb disk is almost up to date when I boot on it.
Does it make sense to rsync the entire file system ? I think some directories shouldn't be copied this way (eg /boot, /dev).

Thanks in advance for help or links.