But. When you select grub to MBR of this troublesome disk, like this:
http://files.myopera.com/carl4926/albums/671478/19.png
When you get back to the install summary, make sure under 'Booting' it is STILL showing the correct HD and to MBR.
Other considerations are:
Are these both SATA
PATA/IDE
or a combination
If both SATA it might be necessary to plug the disk you want to boot from in the first SATA socket of the Mobo. Double check then the boot order.
If a combo, the IDE drive should still be set to Master with the Jumper. Check the boot order to have the drive you need first.