When you were successfully booting the 3B+ from the USB SSD for the last few months, was the microSD card slot empty? If so, was the microSD slot empty when you did the upgrade and is the microSD slot still empty?
I have only been successful in the last few weeks to install and upgrade ubuntu mate and sparkylinux 4.19. I couldn't install and then upgrade any of the raspbian (lite, noobs, lastest buster, etc...) OS. Today I tried to install the last sparkylinux 5 from an usb stick and then an usb HDD but only got a black screen So it looks more like the issue is with some of the latest upgrades files or setup and my raspberry pi 3 b+HawaiianPi wrote: ↑Mon Jul 22, 2019 1:16 amI just tried Stretch Lite on my 3B+ with a USB SSD and had no trouble (already tested Buster, so I know that works).
I wrote the last Raspbian Stretch Lite to a USB SSD with Etcher, booted up my 3B+ and ran sudo apt update && sudo apt full-upgrade -y, followed by a reboot. System booted back up without a problem. Then I used raspi-config to set up my location stuff (country, timezone, etc) and was able to reboot again.
Whatever is happening to you doesn't seem to be a problem with Raspbian. ¯\_(ツ)_/¯
I did the same, installed Stretch on a 16GB SD card, then updated/upgraded. All was OK. Like you I then copied the SD card to an USB-HDD, and also an USB-SSD, and both worked fine. Rebooted a few times, no problem. I hope that when I need to do an update/upgrade it would still be working!!!HawaiianPi wrote: ↑Mon Jul 22, 2019 5:03 pmDefinitely a problem for USB booted systems only. I just tried it with a 16GB micro SD card and had no trouble after the update/upgrade.
Curiously, I was then able to clone the SD card to my USB-SSD with the SD card Copier utility, and it worked! So it's a problem with the upgrade process itself, rather than the updated packages (or so it seems).