I use an encrypted external USB Hard Drive for storage. My PC no longer asks for a passphrase to open the Drive after a reboot or unplugging/replugging the USB cable. The Hard Drive just opens like an un-encrypted drive. The Ubuntu "Disks" tool shows that the external drive is still encrypted. The "Disks" tool also lets me toggle back and forth from Lock to Unlock, but neither setting enables the encryption. It is as if my PC just forgot that the external Hard Drive is encrypted.
I plugged the USB external Hard Drive into different Ubuntu (16.04) PC and encryption works fine. The alternate PC prompts for the drive's pass-code before the Drive can be opened. The encryption issue is specific to one Ubuntu workstation. The PC with the encryption issue is running Ubuntu 18.04 with all the software updates.
lekcin@SFF:~$ apt list cryptsetup -a Listing... Done cryptsetup/bionic-updates 2:2.0.2-1ubuntu1.1 amd64 cryptsetup/bionic 2:2.0.2-1ubuntu1 amd64
In the Disks tool, with the top split screen highlighted, the paddle lock icon indicates that the Disk is locked, while the "Contents" line at the bottom of the tool reads UNLOCKED.
Thank you PerilDuck and Sebastian! I used the "Passwords and Keys" tool (AKA Seahorse) described in the "How to make login keyring forget a password" link provided by Sebastian. That resolved the issue. Now, when I try to open the encrypted Hard Drive, I'm prompted for a password. I also ran "sudo apt install cryptsetup" and it installed OK. Thanks Again.