Ubuntu – OS X no longer visible in rEFInd boot after installing Ubuntu

First, you can set the default boot option in rEFInd by editing the default_selection item in refind.conf (usually in the /boot/efi/EFI/refind directory in Linux). Set any relevant subset of the text that appears when you highlight the option in the menu to make it the default. In your case, default_selection vmlinuz will probably work nicely.

Second, the "two versions of Ubuntu" to which you refer are almost certainly two different Linux kernels. The kernel is not Ubuntu, but it is the core ("kernel") of the OS, and in a technical sense, it's the part of Ubuntu that is Linux -- it's the Linux kernel (as opposed to a FreeBSD kernel, an OS X kernel, or whatnot). In any event, rEFInd is coded in such a way that all the files within a directory are sorted by date within the rEFInd menu, with the most recent file first. Thus, when you install a new kernel package, rEFInd will automatically move it to the start of the list.

This sorting interacts with the default you set in that when you set a default that matches more than one entry (as vmlinuz will if you've got multiple Linux kernels), rEFInd uses the first item in the list as the default. Thus, rEFInd should normally boot the most recent kernel by default if you set default_selection vmlinuz in refind.conf. (There can be exceptions to this rule if you mess with the date stamps by using the touch command in Linux or if you install kernels in a strange order.)

As rEFInd's designer, I created this system to work sensibly with most Linux distributions, provided a reasonable default_selection is set. Sorting by date stamps, in particular, helps a lot with this.

Although refind is good, there seems to be an awful lot missing from the instructions page, it could do with being proofread by someone other than the author so that all the dozens of "what does that mean?" or "where's that located?" questions can be answered right at the start. What's obvious to someone writing the program may not be so obvious to those trying to use it.

Anyway, to find its proper location I managed to find some clues in the uninstall portion of the instructions. These point back to the manual installation guide for instructions on mounting the unmounted EFI partition, et voila!

diskutil list to get a list of partitions, in my case EFI is disk0s1.

mkdir /Volumes/esp to create somewhere to mount the volume to.

sudo mount -t msdos /dev/disk0s1 /Volumes/esp to mount the volume.

Now look in /Volumes/esp/EFI/refind to find your refind.conf file. Alternatively Finder will now have mounted a disk called EFI in your Devices list on the left-hand-side, so you can click on that and browse to EFI/refind.

Googling for the answer found this page so hopefully this little snippet of info will be useful for others trying to track it down.