Okay, yesterday I decided to do the upgrade to Ubuntu Mate 19.04 with a fresh install because I was replacing two drives with higher capacity drives. I replaced a 1 TB SSHD with a 2 TB SSHD (both Seagate), and a 500 GB MSATA Samsung Evo 860 with a 1 TB version. That part was fine. Also, I have another 1 TB SATA SSD for my /home directory. The MSATA drive holds the OS. So, yes, that's a total of 3 drives in a Dell Precision m6700 (which is named behemoth 
).
Okay, how here's the weird part. When I tried to set up the 2 TB drive from the installer, even though I had everything set properly, the 2 TB drive would not be seen, nor automounted. The first time I tried, I thought I had done something wrong, but I had the same result the second time.
I then decided to partition and format the two new drives outisde of the installer. That seemed to work well and much better. Installation went well, and all my drives were accessible. The only weird thing was, again, the 2 TB drive. It was accessible and I could manually mount it. I had to go into /etc/fstab and set up automount there. Once I did that, everything has been working flawlessly. What I don't know is, was there something weird that I did, or was it a problem with the installer. Just for comparison, when I installed Ubuntu Mate 18.10, I had a 3 drive setup in the same machine, and did not have that problem.
The only thing that I can see that caused the problem was the USB flash drive with the installer on it. When I first opened /etc/fstab, it said /dev/sdb was the USB during installion and the 2 TB drive was /dev/sdc but afterwards /dev/sdb was the 2 TB drive.
I think I may have to change the boot order in the BIOS. Sorry for the long diatribe.
Bye, Barbara.