I like that method its a bit weird but seems fine. Essentially you have to put the R Pi setup you want on an SD card and then copy over the contents to your "other drive". Because of some security issue you can't just put a fresh Raspian on the drive on the USB it ONLY works with a copy of what is on the SD card copied over as stated using the command "sudo rsync -avx / /media/newdrive" where you have mounted the other drive in the ".../newdrive" directory. The description on that website is good apart from a few fairly obvious typos such as "sudo nano /boot/cmdlinetxt" instead of "sudo nano /boot/cmdline.txt".I found this easy to follow guide on how to boot from SD then pass control to SSD / HD
https://www.tomshardware.co.uk/boot-ras ... 61081.html
It worked for me for a RasPi 4 although I was just playing around really; I'll use SD only for now.
I do not see any downside to this method apart from perhaps issues once the proper method is working and one has run with the boot directory on the SD card and the rest of the system on the USB drive doing updates. I suppose then to change over care needed to copy "/boot" to the drive in the USB drive.
I intend using a fast 32GB SSD drive for the "system" and a big desktop 1TB for Data and setting up a desktop/server. (Both items purchased at low cost from Amazon and due to arrive tomorrow). For the 1TB drive I have one of those adaptors with an extra 12volt PSU.
So far I have tested using an old laptop 250GB drive and it works OK using the method from https://www.tomshardware.co.uk/boot-ras ... 61081.html
Can anyone see any snags to look out for (now or later)?