Comment 3 for bug 1815225

I remembered some additional information I should have put in the original post:

First: Just to be sure, I wanted to reiterate that the 128 GB USB drive was not just a "boot drive". It was an exact duplicate of the entire Ubuntu system from my laptop, and worked the exact same way (Trash function and all).

Second: As noted, the update from Software Updater that required a restart broke both (1) the GRUB 2 bootloader on the (booted) USB drive, AND (2) the GRUB 2 bootloader on the (unmounted) laptop's hard drive.

When I said they were broken to the point where neither was bootable, taking me to the dreaded "grub_rescue" command line when trying to boot either one, I should have added the following regarding the errors given:

1) When trying to boot from the USB drive (during the restart), the error was similar to "trying to read outside hd0" (hd0 = the hard drive, I believe), before I was put at grub_rescue command line. (To note, I have seen that same error at other *unrelated* times, but at those times, it acted more like a warning pause before simply continuing to the login screen.)

2) Then removing the USB drive and attempting to simply boot up from the hard drive instead, I got an error similar to "cannot find (or locate) device such-and-such" before being put at grub_rescue. I believe the "such-and-such" was the UUID of the USB drive that was being run from when the update that required a restart came up. So even though trying to boot up from the hard drive at that point, it still knew something about the previously-booted USB drive that was active during the restart-requiring-update; and it didn't like that it couldn't find that USB drive, so grub_rescue ensued.

That's it. Thank you again!