I got exactly the same problem as you on my Hi8, at some point it just stopped working.|
First of all I am IT technician so I know a bit what I am doing. But guessing how Chinese manufacturers have set up the software on their tablets is different thing.
BIOS and boot selection menu are both still accessible - tried to boot both (Windows/Android). Nothing - I still get EFI shell. So I tried to fix it by flashing new OS'es.
First attempting to flash android (Android first because I read somewhere that some other Chuwi tablet needed android flash first because android was the base what made up partitons. I may be wrong tho.).
DnX mode and Intel Manufacturing Flash Tool both worked and detected my device - but they could not finish flashing. Stopped at 26% telling me it could not find partition table ...?
Device screen at the failure of flash:
So naturally flashing android didn't work I tried to flash Windows onto it. Flash USB worked well, but it did not finish.
Error I got:
Well, it seems that it cannot detect the eMMC drive. Thinking the problem is with the flashing tool itself - I try to write disk over with backup - found one from 4pda forums. Made a bootable Macrium Reflect USB (it was bundled with backup) and put backup files to root of it. Booted it up - and the Macrium could not find the eMMC drive at all. So I couldn't write the whole disk over either.
Thinking it may be problem with the BIOS, first I tried different options, it did not work. So I flashed it over with BIOS files I found from web. And tried again if the flashing tools and macrium works. Nothing. Same as before.
For me it seems the eMMC chip may be dead or the BIOS/eMMC chip firmware may be bugged. We are not only one with that problem, many have same issue with their chuwi (not only Hi8) and I haven't found a fix for it in the web or tinkering with it myself.
I am thinking of disassembling it and applying heat to eMMC chips (it may be cold solder joint there).
I am waiting for somone from chuwi answer our problems with their devices.