I have a WD 1 tb blue and a SanDisk 120 GB SSD, obviously booting from SSD. I can not for the life of me figure out how to see both of them in the Files. I got close one time but there was an eject button next to my internal HDD. Help is much appreciated.
If your secondary HDD is an external drive you have to first mount it.
Check out this guide.
Open the "Disks" application from the dash.
Select the disk you want to manage. Click the option gear wheel.
If the drive is formatted you can just mount it. Otherwise you will have to format it as NTFS. CAUTION: Formatting erases all data on your drive!
After formatting click the gear wheel again and select "Edit mount options"
Then turn Automatic Mount options off and select "Mount at startup"
The mount point (i.e. folder your drive will be on) you select must be created if you haven't done so already.