Comment 6 for bug 674841

I, too, have "lost" my other Ubuntu partition. I installed 12.04 on sda1 using Btrfs; it booted fine. Then I installed another 12.04 on sda2 (/boot) and sda3 (swap) and sda4 (/), and it boots fine, but now I can't boot from sda1. Mounting and then unmounting sda1 as suggested in comment #2 does not help.

$ sudo fdisk -l /dev/sda

Disk /dev/sda: 128.0 GB, 128035676160 bytes
255 heads, 63 sectors/track, 15566 cylinders, total 250069680 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00076ab2

   Device Boot Start End Blocks Id System
/dev/sda1 * 2048 13672447 6835200 83 Linux
/dev/sda2 * 13672448 14258175 292864 83 Linux
/dev/sda3 14258176 24023039 4882432 82 Linux swap / Solaris
/dev/sda4 24023040 250068991 113022976 83 Linux
$ sudo os-prober
$