Hello!
I have decide to start a new thread because of a new approach I took with my problem (Take one is here: Ubuntu Mate on second hard drive in a laptop).
This time I went with two Mate installs. Both, the internal and in the caddy are unencrypted 64 bit versions. There are 4 partitions: boot, /, swap, home.
I have swapped them after the install just to be 100% sure of my results - on both I did os-prober and updated the grub and both times the end result was the same - the Grub updated fine and at boot I could chose between the two discs but after picking up the disc form the caddy an error popped up which is something new and a big plus for Mate as Mint never showed me that. Maybe this information would be helpful to solve my problem:
error: no such device (here goes a very long string of numbers and letters)
error: hd1 cannot get C/H/S values
error: you need to load the kernel first
Press any key to continue...
Any hints or this is a dead end for me?