I have an uncommon setup. I have Ubuntu installed on one, and a backup from a direct copy of an older Arch Linux install on the second. By direct copy, I mean I tar'd the contents of an entire partition initially, and now I've extracted those contents to another partition. So:
1 - Ubuntu, with GRUB2
2 - Arch from a backup
Ubuntu runs and boots fine. After extracting Arch to a partition, I changed fstab to reflect its new partition. But now I'm stuck trying to get Arch to boot. I tried running update-grub in Ubuntu but it doesn't pick up the Arch install. So how can I get this to work?