Most modern computers, including all Intel-based Macs, can be booted in either of two ways:

• EFI/UEFI mode -- This is the native mode for most modern computers (PCs introduced in mid-2011 or later, and all Intel-based Macs). This is a fundamentally new boot mode from the older BIOS system. It offers both advantages and disadvantages compared to BIOS. The efibootmgr program will work only if you've booted in EFI mode. (Note that UEFI is simply EFI version 2.x. Macs still use EFI 1.x, and so technically aren't UEFI; but 90% of what you read about UEFI booting still applies to Macs in EFI mode.)


• BIOS/CSM/legacy mode -- Older computers have BIOSes, not EFIs. Most EFIs provide a feature called the Compatibility Support Module (CSM), which enables them to launch OSes designed for BIOS mode. This is often called "legacy-mode booting." The CSM is intended as a stopgap tool for backwards compatibility. On Macs, it's often used to enable the computer to dual-boot with Windows (especially Windows 7 and earlier) in BIOS mode and OS X in EFI mode.

Ubuntu can boot in either mode, but knowing which mode you're in can be tricky. The surest way to tell is to look for a directory called /sys/firmware/efi. If it's present, you've booted in EFI mode. If it's absent, you've probably booted in BIOS mode. The mode in which you booted the Ubuntu installer determines whether it installed a BIOS-mode or EFI-mode version of GRUB.

In any event, your symptoms indicate that you've booted Ubuntu in BIOS mode, not in EFI mode. In most cases, this is undesirable. The easiest way to fix the problem is to install an EFI-mode boot loader for Linux. Several are available, as detailed on this page of mine. Traditionally, Mac users have favored rEFIt for this task; however, rEFIt has been abandoned for several years. My fork of it, rEFInd, is under active maintenance. Chances are if you install rEFInd under OS X, you'll be able to boot Ubuntu from the rEFInd menu when you next reboot. You may need to do a little cleaning up afterwards, though:

• Your disk probably has a hybrid MBR, which is an ugly and dangerous hack. It's unnecessary when booting all your OSes in EFI mode, so unless you're triple-booting with Windows, you should probably convert the hybrid MBR to a legal protective MBR. You can do this by making a tiny change with GParted or parted or by using gdisk: Type x to enter the experts' menu, followed by n to create a new protective MBR and then w to save your changes. Note, however, that if you're triple-booting with Windows, you probably need your hybrid MBR.


• You may end up with extra boot options in rEFInd's menu. In particular, there may be a BIOS-mode entry for Ubuntu that might or might not work. You can eliminate it by editing (from Ubutu) /boot/efi/EFI/refind/refind.conf: Uncomment the scanfor line and ensure that hdbios is not among the options. Depending on what else you've installed, you may see other options you don't need and/or that don't work. You can customize rEFInd with other refind.conf options, like the dont_scan_files, dont_scan_dirs, and dont_scan_volumes options. Peruse the comments in the configuration file or read the rEFInd configuration documentation for details.