[ubuntu] Not boot from the second hd

Hi all,
on my PC i have two physical HDs: 160 + 40 GB.
On the first hd i have ubuntu and it is running without problems (i'm using it..).
I installed a Debian distro on the second hd but i'm not able to boot with it.
With different grub configurations i received Error 21 (selecte4d disk does not exist) or Error 15 (File not found).
If i type fdisk -l I see two disks (sda and sdb).
The BIOS shows only one disk (i guess the first disk).

How can i boot on the second disk?

Thanks!