[Galaxy Note 4 Q&A, Help & Troubleshooting] shonkin:
Don't try this at home: my unrepeatable and highly dubious N4 "MMC_read_failed" fix

DON'T Try this at HOME! What follows is *NOT* a suggestion, a solution, or a method of rescuing Note 4 phones with that hideous "ddi : mmc_read failed" error. What it is? One man's absurd approach to fixing a phone that (maybe) worked but was so hit and miss only a real technician would be able to figure out where things actually when right.... *if* they even did. So.....

A friend running a company's phone systems knew I loved my Galaxy Note 4. Yesterday, he handed me another one and said "I can't fix this. I don't think you can. But maybe you can have some fun, and at least use it for parts if you lose your stylus or something."

That's an s-pen not a stylus, but he doesn't use one so.... Onward.

Sure enough, after a few hours monkeying around in vain, I knew this phone was twisted. Odin couldn't touch it; kept saying there was 'no PIT.' Really? I felt myself falling into one...

Did I mention that each time I had to leave recovery (the factory version, not TWRP, alas!) the phone decided it was done working altogether for a while? I at first took to discharging extra electricity by pulling the battery and then holding down all the buttons for a while. Later (some hours, because I am a teeny obsessive) I started putting it in a baggie and then in the freezer.

WHAT?! Oh, yes.... This freezer trick sounds just about as idiotic as anything you (or I!) have ever read on xda or any other geek board. BUT... it does cool the innards so much that it helps marginal equipment run a touch slower. Or so the rationale goes. It did... *ALMOST*... work. Flashed 3/4 of a rom and then the phone blew chunks and Odin sighed and let the session expire.

So. Now, looking at a phone that after all was a spare and probably irredeemable, I decided it was time to go roughshod. If that dang ddi : mmc_read failed message would not leave one way, maybe we'd make it leave another.

I downloaded various "solutions" -- even the ones I knew ahead of time were utter nonsense. Flash this and your Note 4 will not only be well, it will bring about world peace! Yeah, no good result found.

Then I remembered a program called "FRP Tool Pro" -- which for all I know came from Putin's desk drawer -- one I'd used to save another apparently trashed phone... and the way I did that one was also the way I did this one. By flashing a *different* Samsung phone's "soft brick fix" to it. (Hey, this phone was trash already!) The logic here was that if the phone wasn't responding to code meant for it, code meant for a different phone might ignore whatever was blocking the flash and in the process erase it. Or brick the phone, of course... but it was bricked already. So. Randomly, I chose the first one on the list. FRP Tool Pro said it had worked. Weird. Uh, okay... so I did it again, this time choosing the N910T (my Note 4 model). Said it worked again.

Gingerly, I tried Odin again. Again, with the rom, a fail. Tried some other stuff. Fail. Tried flashing TWRP 3.0.2, just for kicks... and it worked. But then nothing else would flash.

Rebooted the phone to try something else, and it went to recovery, and TWRP loaded. What?! Took so long I thought it had hung. But it did finally get there. "Now that I have TWRP on here I'll wipe this sucker clean," said I to I. But no, it said the command couldn't be applied to /system. Which is weird, because /system is always part of a factory wipe. UNLESS....

Unless, thought I with all 3 brain cells, /system has somehow been corrupted or even deleted. And so I went to TWRP's special settings and told it to reformat /system with a different format (there are about five). It happily did so. I made sure to format it back to the original ext4 format.

And now, with TWRP working and the phone finally formatted and reporting no errors, I gingerly stuck in my extmicrosd card with various roms on it. I flashed TWIZtd S8 version, which is what I have on my good / main Note 4, and.... it worked.

I do not think this phone is fully healed. I wouldn't trust it in serious work situations. Tonight I've been playing with it, minus (again) my T-Mobile and Ext-sd cards... just as a little computer via wifi. Works better that way than it does with the cards in it, actually... which is one of the clues that it probably is unwell.

But that said... maybe someone with real sense can figure out if any of this is useful in cases where a note 4 has gone down to the dreaded "ddi : mmc_read failed" message. I'd always thought that meant the phone was toast, sooner or later. I still think so. But maybe.... not?

Remember, though....

DON'T TRY THIS AT HOME!!!