Best thing to do is back up your config (using luci or sysupgrade's backup option) and store it on a another device, then do an upgrade without your stick, and then reinstall extroot and the packages you want on it, and then restore config.
It's not nice and automatic, but extroot is a special case, and there are limited resources for trying to automate the process.
Yeah, my thoughts exactly! Thought it wasn't going to be easy.
IIRC I was very limited in space on the original install whereby if I used a standard image, there wasn't enough room left for any USB support which is required for ExtRoot.
I think I might have to bite the bullet and create a minimal image without LUCI, ppp, IPv6, but to include the USB packages..
Surprisingly, I took a look at 18.06.0 today and the mr3420v2 isn't supported. Oh well I suppose I might get a while longer with this router on 17.01.5!
Oh, and it might be supported in .1 - I think in the 'known issues' there was some mention of an issue affecting certain boards that would be back for .1