Two SSD´S two OS´, only one will boot.

You are hitting a known issue!

The optical bay SATA port logic on the logic board has a defect so it won’t run at SATA III (6.0 Gb/s) it will only run at SATA II (3.0 Gb/s). So when you put in too fast a drive in the bay (SATA III 6.0 Gb/s) the drive over saturates the port with data.

Think it like trying to tell your deaf grandparent a story… They can only hear the first few words (2) then they can’t hear anything more, they say what?? You repeat your story they manage to get the third word and then they say What! You repeat again this time they get the fourth and fifth before they shoutout WHAT!! And then they just shout down.

Thats about what is happening here! Your system is able to buffer up a few blocks before it get saturated. If the block group is small enough it manages to pass the CRC checksum and get moved into RAM freeing up the buffer for the next group of data blocks. Here we are talking about tens of thousands of blocks!

OK, how to fix? You need to replace the optical bay SSD with a fixed speed SATA II (3.0 Gb/s) drive.

OWC did a great write up on the issue in their OWC Data Doubler doc. Locate the MacBookPro8,1 listing in the compatibility listing note the Red note.

To be clear: You need a fixed SATA II (3.0 Gb/s) SSD not one that auto senses the I/O speed like a Samsung drive as it won’t correctly sense the needed I/O.

The other option is getting a larger SSD so you can run both OS’s in independent partitions within the HD bays drive.