Yeah the one in the cartridge. You would probably see a change in the light on the SP or DS if you have one of those on the way out.
On writing 256Mbit files then https://ezflash.sosuke.com/download/file.php?id=674
Copy it to your cart and write it to the NOR a few times. That might solve the issue if it is a sector on the way out that the chip needs to reallocate.
The NOR fix I spoke of there was in the later loaders -- some people were having issues with their NOR failing to write the last megabit and it freezing at 255. As this meant mother 3 did not work (most other things trim or can be trimmed) you can imagine the fun that resulted there. The EZteam then released a fix in a loader up for the EZ4 that seemed to sort it. With that said the EZTeam have been seen to play a bit fast and loose with error checking before and that might have happened here, though I don't know what was the cause or the fix for that matter.