Accessing one encrypted boot drive from another in 18.04
I recently made the jump from 16.04 to 18.04. I did a fresh install, including encrypting my entire drive, but now I am having problems that I did not have in 16.04.
As a little background, I have an external USB HDD from which I run Ubuntu on a desktop. I also have a laptop which uses the internal HDD. Everything was working great with 16.04. That is, I could boot, run, everything was fine. I could connect my USB HDD to my laptop, give the encryption password, it would mount, and I could copy files between the drives. Remember, all of the drives in question are encrypted boot drives.
Then I installed 18.04 on my laptop (encrypted drive). Worked great, even after I changed the volume group name.
Sadly, I can no longer connect my 16.04 HDD to my laptop (laptop is running 18.04). I get the small, unencrypted, boot partition to mount and I see the main encrypted partition in the sidebar, and I even get prompted for the password when I click on the main partition. However, once I enter the password it says that I cannot mount that drive and the drive disappears from the sidebar (I am doing all of this with the GUI).
I installed 18.04 on another USB HDD and my 18.04 laptop cannot mount the 18.04 USB HDD either. My 16.04 USB HDD can mount the 18.04 USB HDD. My 18.04 USB HDD cannot mount my 16.04 USB HDD.
Short version: Even when everything has unique a volume group name, 18.04 with full disk encryption seems no longer able to mount another boot drive (16.04 or 18.04) with full disk encryption.
What could I do to fix this?