How to set up to boot OpenPlotter from an SSD disk

[LarsD]

Hi,
I'm coming towards the end of my OpenPlotter build (yes I will document and post when done) but I have one major hurdle left.

I have incorporated an SSD Disk over USB in my build and want to boot from that one instead of from the SD card (yes I know how to tell my RPI-3 to boot from USB).

I downloaded etcher (to my PC) to write an image to the SSD (I think I've understood how to do that).

My challenge is that I need a valid image and Etcher refuse to use the OpenPlotter download that I also copied to my SD card (as per a normal OpenPlotter install). I assume that isn't a plain image so that's why it fails.

I have just installed a fresh OpenPlotter to my SD card and am running the update to the latest version right now. Once this is done, can I copy that image any way to my SSD or how should I go about doing this. I'm totally lost unfortually.

Can I do this on my PC (I'm no Linux wizard to say the least...) or can I mount my SSD (currently fresh without any file system) to my RPI and copy my SD image to the SSD - HOW??

Does anyone know the easiest way fwd??

BR
Lars