How to grub-install ignore specific drive/partition

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http://askubuntu.com – Is it possible to use grub-install or update-grub to just search on specific disk/partition? (or ignore specific)? I installed Ubuntu 12.04 on my hard drive, but i wished to do some testing on it without harming current installation, so I "rsynced" root partition (the only) to the USB partition (ext4). I did fix /etc/fstab on USB partition. The problem is that when I do grub-install /dev/sdb (usb) GRUB seems to confuse when UUIDs. Whatever I chose in GRUB it always boot from disk. In grub in edit mode I see that in two "UUID" lines are not the same. If I retype UUID from "first" to second "li (HowTos)