Did you have this problem with the USB drive before adding the extra disk for the plexfiles? (sdb1 in your fstab)?

It is really not really the best practice to mount disks using using the dev/sdxy device name; /dev/sdb1 in your case. Disks can 'move'. Wonder whether the USB drive, when inserted before boot, is becoming /dev/sda1 or /dev/sdb1 and hence you are dropping into emergency mode as it cannot perform the mount as per your /etc/fstab as your 'plex' disk is no longer /dev/sdb1. It is much preferred to use the UUID. Your fstab is uisng UUID for the boot drive... If something like this is occurring - then the instructions below should help...

If you run ' ls -l /dev/disk/by-uuid ' - you can see the UUID for your disk partitions...
Check that you can see the one for your /boot partition to verify that you understand the output, matching it to your fstab ... suspect it is /dev/sda1
Then find the UUID for /dev/sdb1 and amend your fstab - using the entry for /boot as a template for the format...

Check first without the USB drive - then reboot with it inserted? Fixed? If so, is the USB drive still /dev/sdc1 when you boot with it inserted? If it moved then checking /var/log/messages is one of several ways to find its new device name...

There are other causes for your problem - but this is the first solution that should be attempted...