Second internal drive not initialised

The cable also gets damaged on the top ledge. I use 3M extreme mounting tape to protect it.

Some Macbook Pro’s have an issue with the Optical drive SATA port. While the 2012 models had been fixed I wonder if the newer Samsung drives interface driver has a problem. You may need to update it. The 870 EVO Plus had a driver issue which Samsung has fixed.

Let’s try this take your current 840 EVO out of the HD bay and put your new 860 QVO in its place. Are you able to install macOS onto it via Internet recovery?

If that works that is pointing to an issue with the Optical drive SATA port.

In anywise do make sure you have the newer HD SATA cable as its known to have issues! Place some electricians tape on the uppercase where it crosses over and don’t crease the cable! Instead create an arc using a BIC pen ink straw (or similar ink straw, empty) to help form the radius.

I haven’t encountered issues with the Optical drive’s SATA cable but you might want to give that a try as well if the HD bay works.