Ubuntu – How to return to OS X after installing Ubuntu

Try holding down the Option key (or Alt if you're using a PC keyboard) when you hear the chime early in the boot process. That should produce a boot menu that will enable you to boot to OS X. You can then re-install rEFIt (or switch to rEFInd, which is my updated fork of rEFIt, since rEFIt hasn't been updated in over two years). This should fix the problem.

If using Option or Alt doesn't give you a boot menu, try using a rEFIt or rEFInd boot CD. (Both projects have them, but the rEFIt CD is more likely to work on a Mac.) Again, you might need to hold down Option or Alt to see the CD as a boot option. Once booted, you can (re-)install rEFIt or rEFInd.

If neither of these approaches work, I see three others, but they all carry some risk:

• You can move or rename the /boot/efi/efi/ubuntu/grubx64.efi file from Ubuntu. This is GRUB, and moving/renaming it will, with any luck, force the Mac's firmware to look for other boot options, and it should find the standard OS X boot loader. Once you're in OS X, you can mount the ESP ("mkdir -p /boot/efi && sudo mount_msdos /dev/disk0s1 /boot/efi"), move GRUB back to its normal location and name, and (re-)install rEFIt or rEFInd. The risk is that if the firmware can't find the OS X boot loader for some reason, your system won't boot at all until you use an emergency disc to correct matters, or maybe even until you re-install an OS.
• You can do much as above, but copy the rEFIt or rEFInd files into /boot/efi/efi/ubuntu, renaming refit.efi or refind_x64.efi to grubx64.efi. When you reboot, rEFIt or rEFInd should appear, and depending on which you use and what you do with the original grubx64.efi, you might even be able to boot both OSes right off the bat. The drawback is that subsequent GRUB updates might wipe out your changes.
• You can try using the efibootmgr program to adjust your EFI boot loader configuration to move rEFIt back to the top spot (or to give rEFInd the top spot, if you try to install it). The risk is that efibootmgr has been reported to corrupt the firmware on at least some Mac models. My impression is that this problem has been corrected with post-3.0 kernels, but I haven't found anything authoritative on this. In any event, if you try this, either read the efibootmgr man page and issue the necessary commands to reset rEFIt to be the default, or download rEFInd and use its install.sh script to install rEFInd, which will also use efibootmgr to make it the default boot loader.

Good luck!

The answer is "it depends."

If your GRUB is an EFI version of GRUB, then in theory it can replace rEFIt. Doing so on a Mac is a bit tricky, though, and I don't have step-by-step instructions for doing the job.

Most Ubuntu installations on Macs install Ubuntu in BIOS mode, though, and therefore use a BIOS version of GRUB. In this configuration, both boot loaders are (more-or-less) necessary; rEFIt is unable to boot a Linux kernel directly without jumping through awkward hoops, and the BIOS version of GRUB can't boot OS X.

An alternative you might want to consider is replacing rEFIt, which is no longer being developed, with its fork rEFInd, which is. Unlike rEFIt, rEFInd can boot a Linux (3.3.0 or later) kernel directly, so by installing rEFInd, you can bypass GRUB. The procedure to do this would be:

1. Install rEFIt from OS X by using its install.sh script.
2. Manually install the driver for whatever filesystem you use on Linux's /boot partition, or on the root (/) partition if you don't have a separate /boot partition. See the rEFInd documentation for details on its drivers. (You'll just need to copy one file.)
3. Reboot. rEFInd should appear and show you an option for OS X and at least one for Linux (with a penguin), and one for GRUB (with either a generic icon or an Ubuntu logo). Select the one that boots via GRUB.
4. In Ubuntu, locate the mkrlconf.sh script that comes with rEFInd and run it with sudo. This creates a configuration file that helps boot the kernel directly.
5. Reboot. You should now be able to launch the direct-boot option(s) in rEFInd. (The ones with the Linux penguin icons.) If you have problems at this point, you'll have to resolve them, or maybe just use rEFInd as you'd been using rEFIt before.
6. If everything's working, you can optionally remove the old GRUB entry from rEFInd's menu as follows:
   • If you'd been using a BIOS-mode GRUB, locate the refind.conf file (on the ESP or the OS X / partition, depending on how you installed rEFInd), uncomment the scanfor line, and remove the hdbios option from that line. Note that if you also boot Windows, though, this will remove Windows from the boot options, so you probably don't want to do this if you're triple-booting with Windows.
   • If you'd been using an EFI-mode GRUB, you can either remove its files (probably in /boot/efi/EFI/ubuntu) or use the dont_scan_files or dont_scan_dirs option in refind.conf to restrict scanning for GRUB.