This has become somewhat confusing. I believed I had a thumbdrive as the exroot partition as /dev/sda1 and the media storage hdd as /dev/sdb1. Now they read in reverse. And neither appear enabled in fstab.
So that I don't harm this, what do I need to check to see if the exroot is working?
You can change the location of mount points however you want. /mnt/sdX are purely from block detect and don't need to correspond to the drives /dev name. At runtime the partitions are referenced by their UUID, which is written on the partition when it was formatted and won't change as hardware is moved around. I usually create a /data or /srv in root and mount external drives under that, leaving /mnt empty for temporary mounts.