ok. try this since you can at least boot:
- Get an 8g USB stick, format it for Extended (journaled) and name it Untitled. Case matters
- Download High Sierra to the Applications folder.
- Copy and paste the terminal commands from this Macworlds article. This is why case-sensitsty matters. It looks for usb drive named Untitled
- Go make dinner. This takes a while.
- Install the SSD drive into the main drive bay; not where the optical is. Close up the computer. You don't have to do this, but I don't like even resting the computer on the bottom case.
- Boot with the USB drive inserted and hold down the option key. Have no other USB drives inserted
- You should see a boot screen where you can choose from the High Sierra installer
- I forget the next screen and the exact menu choice, but on one of the menus you can choose Disk Utility.
- If all is well, you will see the SSD and can format it to Extended (Journaled).
- Quit when done
- Install High Sierra.
- Reboot and set up the user account on the main SSD.
- Once you know everything is working ok, install the Optibay I am assuming you bought with the second SSD drive installed.
- Format the SSD drive.
I don't recommend putting your home drive on the second drive, and instead using it just for general storage. I did notice the two drives in my old 2011 impacted battery life. This way if you change your mind you can just pull the bay out. I also recommend just leaving it in there any buying an external Optical drive if you need one. I've found the ribbon cable to be pretty flimsy.