USB booting :: Bott from USB-HDD and Local HDD

Hello,

can one help me with this wish :
I have a local (build-in) hdd and a USB-HDD.
On the local HDD there is Windows and SUSE Linux with GRUB on it.
On the USB-HDD there is DSL.
I would like to have the following :
In case the USB HDD is not connected then GRUB from the local HHD sould come up and let me choose between WIN and SuSE.
When the USB-HDD is connected then the USB-HDD-GRUB should come up and let me choose between DSL, WIN and SuSe.

Can one give me some hints howto manage ?

thx
Frank

Why not just create a menu for WIN, SUSE and "DSL - External USB" and then only decide to select the last choice when your drive is plugged in?

original here.