If anyone is still interested in fully disabling the EMMC on the 5070 celeron/pentium/extended_pentium/5470AIO, here is how to do it:
Download the BootX64.zip shared here.
Using RUFUS or some similar tool, make a bootable USB and replace the X:efi/boot/BootX64.efi on the newly created USB, with the extracted BootX64.efi file
Power on your 5070/5470 and press F2 to access BIOS settings
Default Password is Fireport
System Configuration > USB Configuration: Check Enable USB Boot Support
Secure Boot > Secure Boot Enable: Uncheck Secure Boot Enable
Hit apply on bottom right. Doesn't matter if you save as custom settings or default.
Make sure your Bootable USB is inserted and Exit
press F12 while booting to access boot menu and Select your USB
In GRUB type: setup_var 0x13a 0x00
Press enter, you will get some warning, proceed with caution etc.
You're done! press ctrl+alt+del to reset, and remove USB.
0x13a is the hidden variable to disable EMMC, 0x00 is Disabled 0x01 is enabled.
So setup_var 0x13a 0x01 will re enable the EMMC
Also, restoring BIOS defaults will re enable the EMMC, and in some cases, BIOS updates will also cause it to re enable so make sure you update the BIOS before disabling, or keep in mind you may need to disable again later.
If you have a Windows 10IOT license, the product key is stored on the EMMC, but once your windows install on the m.2 ssd is active, you can disable the EMMC.