I have searched the forum, and it seems that other people have run into this.
During installation, the installer asks you to pick a hard drive for the system to be installed onto. You must pick one, or another hard drive, you cannot pick two or more drives.
Raw rookies aren't likely to want to use more than one drive - in fact, many of them want to keep a Windows installation, and dual boot to their new Linux installation.
Others of us, with a bit more experience, have various reasons for spreading an installation across several partitions, located on two or more hard drive. In my own case, I've got a brand new SSD hard drive that I want to use as my system drive, but I want the swap, tmp, and var files on an older hard disk with spinning platters.
Despite the fact that I had already partitioned the older hard drive, labeled and formatted the three partitions, the installer would only allow me to choose the platters, OR the SSD - not both. During installation, there is no option to relocate any of the folders, and no opportunity to tell the installer that I want any partitions mounted automagically for me.
I'm searching for methods to automount partitions from that second hard drive - but I really think the installer needs to be made more flexible, to better meet people's needs.
I gave a thought to using an LVM setup, but I'm unsure whether Mint will support it. More, I'm unsure how LVM will position those partitions on the drives. The goal here is, of course, to prevent often-changed data from being written to the SSD, thereby extending the life of the SSD.
Hope something can be done with the installer!!