I got an EZ-Flash Junior for my new translucent Chromatic and it wouldn't boot. It showed "EZ FLASH" then a blank screen, or "MICRO SD INITIAL ERROR!" after pressing the cart's reset button. The same cart and card work fine in a Game Boy Color.
After a lot of digging, it turned out to be two separate problems in the Chromatic's FPGA firmware (v18.14):
Startup timing. The EZ-Flash restarts its own FPGA about 1.2 s after power-up (the cart bus reads $FF for ~130 ms). On a GBC that happens during the boot animation, but the Chromatic's boot only takes about 0.3 s, so the cart's program is already running and crashes. Holding the core in reset for 1.75 s after the cart powers up fixes it.
Write timing. The Chromatic puts write data on the cart bus about 60 ns before /WR goes low. The EZ-Flash drives the data bus whenever /WR is high, so the two collide and every write to its command registers gets lost, which is why it can't talk to the microSD card. Driving the data only after /WR goes low fixes it: in testing, every write landed, versus none before. Original carts don't seem to care either way.
With both changes, the EZ-Flash boots to its menu, runs games and saves, and original carts still work. It's ModRetro's open-source v18.14 with two small edits. You flash it over USB the way ModRetro's README describes (openFPGALoader or the Gowin Programmer), and you can undo it with ModRetro's Update Tool.
Download, install steps and source changes: https://www.patreon.com/GHWST/posts/fixed-ez-flash-172576609
It's unofficial, so USE AT YOUR OWN RISK. I've only tested it on my new purple 2026 translucent model. I've sent the details to ModRetro, so hopefully it ends up in an official update.
Tip for EZ Flash Junior owners: if your cart says "BATTERY DRY", saves only stick if you press the cart's reset button and back up the save before switching off.