I had a scary moment after I updated my dual boot MacBookAir to Mavericks. Refit started Ubuntu, but grub was showing me the rescue prompt. This was a different misadventure from my upgrade to Mountain Lion; the partitions were there, but grub was lost.

I use refit to manage dual boot. I remembered from past experience that I needed to use the partition tool from the refit boot screen to sync the GPT and EFI tables. The task was done in seconds.

I knew Ubuntu was on the 4th partition (suggested by the GPT output from the sync), but to be sure I listed the partitions, then listed the 4th one.

ls
ls (hd0,gpt4)/boot

and I saw the boot images and a grub dir. I then assumed, all my data was in place, but the MBR was lost during the Mavericks install. To start Ubuntu, I typed:

set root=(hd0,gpt4)
set prefix=(hd0,gpt4)/boot/grub
insmod normal
normal

Ubuntu started up as normal. The grub screen was identical to my last boot. I was confident grub's configuration was fine, I just needed to restore grub to the MBR. I opened a terminal after logging in and typed:

sudo grub-install --force /dev/sda

With hesitation, I rebooted. All was back to normal.