I just did this on my Ugoos AM6 Plus
I (nor CoreELEC) guarantee it will work on those other boxes, but I can testify it works really well on my Ugoos and don't see why it won't work on the others too.
And of course it frees up your SD Card Slot for other use
Here is the guide on CoreELEC forum. - https://discourse.coreelec.org/t/how...emmc-tool/7630
You can easily copy your existing SD Card CE config into a newly created partition all in one step, without affecting your Android at all; and when you want to update your Android firmware it is just as easy to remove it and re-install after upgrade all using SSH
As per my linked post you just SSH in to the CE-booted OS and execute CEEMMC -x (the program is already incorporated on NG CE systems) - and it will literally walk you through it from there without even needing the guide (although the guide does give you an advance look at the options descriptions)
To move your CE into a Dual-Boot set-up you would select option 1
When upgrading your Android, you would use option 6 to copy it back to an SD Card, then option 7 to delete it from the EMMC.
The only thing missing is that from power-on it will boot to CE and from there you would reboot to Android - that is no different from the external SD Card install however (unless you pull the card out of course).