[T-Mobile Galaxy Note 4 Q&A, Help & Troubleshooting] notufatjesus:
I have seem to have found the issue and solution to this problem that many of us have been experiencing. If you have encountered issues like myself, there is a thread dedicated to the description of this problem; MAJOR LAG, RANDOM REBOOTS, the phone won't boot up and goes into a screen where you read:

----Mmc_failed failed boot---

After searching for about a month, it seems to be an internal SD issue. Many have reported to having these issues after they root, others upgrading to the non reversible DOK2 FIRMWARE. Others were stock with out root and had this issue as well.

**THE PARTITION OF THE INTERNAL SD CARD IS CORRUPTED**

Could be corrupted files, that stick to the phone EVEN AFTER ODIN FLASHING STOCK MANY TIMES.

(I Odin 5 times back to back to fix and the partition was still corrupted)

People thought "well, if Odin doesn't work, it's a hardware issue" I thought that myself, and it was not.

My friends, I have found something that has worked for me and would like to share with you guys. I hope this fixes your device like it did mine. I've been so close to throwing this Note 4 against the wall, running it over with my car, or smashing it with a hammer. I had no way to return my device as many have suggested to do because I left T-mobile a while back. Plus, rooting my phone voided my warranty. Yada, yada, yada, let's get into it.

*Since the internal partition to the SD card is messing up, we need to repair it.

**In order for this to work, you will need to be ROOTED.

*Download an app called "AParted SD Card Partition." Hope I don't violate any rules, but here is the link to it on Google Play.

https://play.google.com/store/apps/details?id=com.sylkat.AParted

Once the app gains root access, you will be brought to the main screen. Click the tab that says tools. Once that screen pops up, you will see

Part 1 FS: fat32 . Size 31914 MB

Part 2

Part 3

Part 4

Please note that the size to Part 1 may be a different number. That is what mine read.

*You will now click Part 1

*After doing so, at the bottom where it says create, click it and replace it with REPAIR.

*Hit apply, and you're all set.

*Turn off phone, pull battery out for 10 seconds, as well as external SD card and Sim. (They hold small amounts of electric current which may prevent the phone to do a proper boot) I usually hold both volume buttons, power button, home button, and pull the s spen out for 10 seconds simultaneously, then put it all back together and boot up.

*******Also, I used the app to delete the partition, and creating it again to avoid problems from a faulty repair. So if you want to do that, click Part 1. Instead of repair, click delete. Once done, click Part 1 again, then click create. Hit the Fat32, and start it. Once it's created, you shouldn't have issues. In case you're worried, pictures and videos will not be erased.******

I HAVE NO AFFILIATION WITH THE DEVELOPER, BUT HE SAVED MY DEVICE WITH THIS APP. PLEASE, PLEASE, PLEASE DON'T FORGET TO THANK HIM AND HIT THE LIKE BUTTON IF THIS WORKED FOR YOU AS WELL. LEAVE COMMENTS AND QUESTIONS BELOW, AND IF YOU SEE PEOPLE TALKING ABOUT THE PROBLEM IN OTHER THREADS, RE- DIRECT THEM TO THIS THREAD.