Ubuntu entry not working even if GRUB is correctly installed on target drive


I've installed Ubuntu on a e-SATA HDD. I currently boot into it by pressing F8 at startup and accessing my motherboard's boot selection menu. GRUB is correctly installed (and working) on that HDD.
I would like to add an entry for this Ubuntu installation under Windows 7 bootloader which is installed on a RAID drive (obviously completely independent from the HDD where Ubuntu is installed).
I tried adding an entry for Ubuntu under Add/Remove Entries... I selected the correct partition, added the entry and rebooted. It does not work at all. If I choose "Ubuntu" entry under Windows 7 bootloader, I get an error message of some kinda loader like "operating system not found"...
What can I do to fix it?
Thanks in advance