Recently I moved all my media files from my old USB HDD, which was probably always a bad idea, onto a new 1TB USB SSD. I decided to use the old HDD for backups. Unfortunately during the first real backup, everything failed. It took a while to get to the bottom of it, but it seems Plex filled up my 128GB flash card. I had assumed that would be enough for my files and Ubuntu, but I guess not. It seems lots of temporary files ended up in /var.
When I started this experiment in desktop Raspberry Pi there wasn’t much of a choice. You had to boot from the flash card but you could mount a USB drive. So that is what I stuck with, with my media (photos, videos, etc) on the external USB drive and Linux and home directories in the flash card. Well a year on it seems you can boot from a USB drive, so I bit the bullet and decided to move everything to the USB SSD and boot from it.
After a few false starts (you can’t use Ubuntu 20.04, only 20.10) and lots of time copying files around I got it all to work. Advice for anyone doing this today: go straight to a USB SSD and ignore the flash card. There is a nice utility to load up any OS of your choice from any popular platform (Windows, Mac, Linux). You can be done in a few minutes. The instructions on line require some updating of the Raspberry Pi 4 firmware but I found that wasn’t necessary in my case (maybe I had already updated it). Pretty impressive.