Help with grub and external drive..

Let me clarify a little thing first.. I'm a total noob with linux, but considering my field of study i guess i can give it a try..
That being said... I've installed debian (testing release) on my acer 5920g, but due to disk space issue i had to install it on an external hard drive..
The installation went flawlessy, and after little to no time i had all debina configured (or at least i hope so).
This may sound like an happy conclusion but here the problem starts..
Whenever i start my notebook without the external hd plugged in, Grub throws me an error 21, that i suppose is something connected to the mbr being installed on the main hard disk and the missing reference to the usb hd that is not plugged in..
Whilst i suppose i could always try to turn on the notebook with the usb hd plugged it when I'm at home, when i'm at the university or when i'm at away it really isn't an option..
Granted i could repair the mbr and get back my old windows mbr, my little question is...
Is it possible to make the "new" mbr stay totally on the usb device, hence being loaded ONLY when the external hd is plugged and, whilst one it isn', the "standard" MBR will be loaded?
Thanks in advance for any help.

P.S. I tried browsing google for any help, but while i got some answers i preferred not to take any step since i'm not proficient with using linux and i'd like to avoid any trouble with my hard disk data :P