On one of the systems, I experienced the problem, that upgrade-from-grub-legacy could not install GRUB. When I tried to install GRUB manully using grub-install, I got a strange error message:
/usr/sbin/grub-setup: warn: Your embedding area is unusually small. core.img won't fit in it.. /usr/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and its use is discouraged.. /usr/sbin/grub-setup: error: if you really want blocklists, use --force.
It took me some time to figure out, that this system had an unusual disk geometry, resulting in not enough space being available before the start of the first partition. The output of fdisk -lu /dev/sda looked like this:
Disk /dev/sda: 320.1 GB, 320071884800 bytes 64 heads, 32 sectors/track, 305244 cylinders, total 625140400 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: 0x0009153c Device Boot Start End Blocks Id System /dev/sda1 32 585727 292848 83 Linux /dev/sda2 585728 625139711 312276992 5 Extended /dev/sda5 585760 215429119 107421680 8e Linux LVM /dev/sda6 215429152 430272511 107421680 8e Linux LVM /dev/sda7 430272544 625139711 97433584 8e Linux LVM
Usually, the first partition starts at sector 63 (for systems installed more recently, 2048 is also common), but for this system it started at sector 32. I guess that this was cause by the strange geometry, which did not specify the usual 255 heads and 63 sectors/track. I am not sure, whether the disks (two Western Digital 320 GB drives) really have this strange geometry, or the RAID-Controller, a 3ware Escalade 8006-2LP, reports a different geometry. However, for sure the first partition being to close to the start of the disk was the cause of the problem with GRUB.
Unfortunately, I could not move the partition using parted and gparted does not allow to specify the start and end of a partition in sectors, but only in MiB (and only supports aligning to MiB or cylinders, which did not help considering the odd geometry). Therefore I decided to delete the partition (after all it was only the boot partition and its content could easily be stored on the root partition) and create a new partition using fdisk.
In fact, when (re-)creating the partition, fdisk suggested to start the partition at sector 2048, which I accepted. No surprise, after that change GRUB could be installed to the MBR without any problems.
The good thing about this problem was, that the time I invested to get a high-availability setup running for most important services, finally paid of. I for sure would have been less relaxed, while figuring out and fixing the GRUB problem, when the services usually running on this machine would have been offline.