Adding a new disk makes my server unable to boot

Issue

Adding an other disk makes the server unable to boot. The initial configuration looked like this:

• boot from controller 1
• /boot and the OS is on controller 2
• controller 1 and 2 both contain one (logical) disk
• grub.conf lists "root (hd1,0)"

After adding an other disk the configuration has been updated to:

• add additional disk on controller 1 (disk two)
• still boot from controller 1 (disk one)