Yes. But there are potential problems with the grub bootloader. If you write
grub to the hard drive and change your setup you may not be able to load
Ubuntu. The bootloader is a snapshot of your system at a point in time using
relative drive designations. When you change devices before booting it could
change the relative designations.
My preferred method is to write grub to the external device and then use the
boot manager that is built into the netbook to select the boot device.