It's been a while since I've been here. I apologize for my ignorance, but i'm new to EOS/Linux. And i thought i'd post here in the EOS forum, rather than a more general one. I haven't had any response on another forum specifcially for EOS.

I had to reset the PRAM on my MB Pro the other day. I had EOS installed, and it would automatically boot to it (still hadn't figure out how to get the GRUB screen up). I would have to hold down the option key to get the choice to boot El Capitan.

Anyway, after the reset it automatically boots into OS X. I thought i had erased EOS but when I attached the USB stick in order to reinstall it, it noted that EOS was already on the machine. I didn't install and took a look at GPARTED, and sure enough there it was, with the swap file and everything. In OS Disk Utility it showed the mac partition and the other partition for EOS, but was called 'unidentified.'

So to my mind there has to be a way to be able to boot into EOS, but I don't know how to do this. Can someone help me out here? If you need more info let me know.

Here's what I can tell you so far after some replies:

I just turned on the mac after inserting the usb stick with EOS on it (i think its an iso file). When I hold down the option key I get either the mac partition or the usb stick (which is called EFI boot). I choose that one, and now I'm looking at the GNU Grub screen, with the the choices to

Try elementary OS without installing
Install Elementary OS
OEM install (for manufacturers)
Check disc for errors

There's no rescue here (which i've been advised to try) obviously. So if I choose install, which I think is what you mean, then I get the welcome screen with a language choice

Then when I click continue I get the preparing to install elementary; i click continue with a choice to download updates while installing and one to download third party software for graphics and wi-fi hardware, etc. I don't choose either and just click continue.

Then I get the installation type window. at the the top oft he window there is a message saying that EOS is already install the computer.

I choose the "something else" option, rather than erased the disk, or encrypt the new elementary installation for security, or the use the LVM with the new elementary installation.

Then I get the installation type with all the the partitions on the SSD:

/dev/sda1 efi
/dev/sda2 hfs+ (the mac partition]
/dev/sda3 hfs+ (mac recovery partition or disk)
/dev/sda4 hfs+ (another mac recovery disk)
/dev/sda5 ext4 (the EOS partition)

There is also the linx swap file

There is also "device for boot loader installation" with the name of the SSD in the mac

There are no mount points for any of these partitions, if that makes any difference.

If a cancel the install I get the desktop, and when I open gparted it shows all the partitions, but the only one that has a flag is the EFI System Partition. none of the other partitions have any flags, and I'm not sure of that means anything.

I might not be understanding you. I used something called Boot-Repair-Disk https://sourceforge.net/projects/boot-repair-cd/
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt update
sudo apt install boot-repair

It is highly doubtful that OP will return to this topic after 4 years.
They last logged in here over a year ago.