Question: Having the same issue
I'm having the same issue, I'm running 2 x 2TB Samsung 850 EVO: The one in the SATA drive is recognised properly but the one installed in the Optical drive can not be unmounted and formatted. I have already bought 2 new SATA to optical drive cables with no luck.
I'm running a late 2013 unibody MacBook Pro on the latest version of High Sierra. Samsung tech support have updated last week the disk firmware but after receiving the disk is still not recognised.
The Pro Dude, where you able to install and format your disk at the end?