I've partitioned the drive on my Mac Mini 5,1 so that it's half MacOS Mountain Lion and half Ubuntu. I had it dual-booting for a little while with rEFIt, but when I reinstalled Ubuntu from a USB live disk, it wrote Grub to the /dev/sda boot area, and now I'm not getting a mac boot screen at all, and the grub options to boot into MacOS don't work. How can I restore the rEFIt (Mac) bootloader, so that I can boot back into MacOS?
Easiest way would be to boot into OSX then run.
See refit install change enable.sh to enable-always.sh. This will bless the drive and efi startup file. Hold down X on boot might force OSX to boot. See boot select. If none of that works boot from OSX CD and then run the script that way.
there is also something called mactel I'm not sure if its in the ubuntu repositories or not and I have not used it yet myself. mactel could in theory bless the drive and file from linux.