I'm adding ANOTHER ANSWER to answer your OTHER QUESTION, which you added to your first question after I offered my original answer, chosen above:
Update: Thanks so much for the thoughtful responses! At this point,
I'm wondering how I should go about keeping my system backed up to a
bootable external drive.
What you are asking can be answered different ways depending on what you are trying to do. I can think of four possible ways, I will explain in outline format below:
1) Synchronize data-only across two bootable drives:
This seems to be what you want to do, but I believe you will run into a problem which will impede data security. I will explain.
It seems to me that you seek to SYNCHRONIZE data files between two (or more) bootable operating systems. Though I've never actually done this, I know that it is only a google-search away, and very likely explained on this site. However for that to work, you'd almost have to turn encryption off, or rather, never turn encryption on. In my opinion, if you are going to have portable bootable external drive, they should always have data encryption turned on, so unless you discover a drive synchronize that can also accept password data, or unless you decide to travel without data encryption, this is not a viable solution.
2) Clone again the internal or external drive in its entirety to another drive:
Use the "Clonezilla" utility, mentioned in the very last paragraph of my original answer, to clone the original internal drive to another external drive to make two such external bootable drives to keep track of.
3) Use the backup utility included with Ubuntu with your external bootable drive:
This is what I do. I boot (nearly) ALL MY COMPUTERS to a single external drive which travels with me, and when I am home, the 128gb slow usb flash drive, which remains plugged in my home computer, makes incremental backups of changed files on the traveling boot drive.
In your case, I would recommend that in the interest of data security, that you physically remove your internal hard drive and leave it at home. It will frustrate the heck out of anyone who steals your laptop, in fact if they get to try it before they steal it, they might even decide it is broken and leave it alone. If the thought of having only one bootable external drive frightens you, bring two and mark the spare clearly as such so it is never used unless the first one is lost or destroyed. As long as you create the backup drive on the first external drive before cloning it to make a second external drive the incremental backup drive will continue to work with the spare bootable drive.
4) Use the backup utility included with Ubuntu in the traditional method:
Use the backup utility to backup just the data from the internal drive to an ENCRYPTED data-only external drive. You can configure it to make "incremental" backups of any changed files on your internal hard drive to the external, non bootable, SSD.