[XDA Assist] cmcarthur:
I have looked everywhere for a solution to this problem and found a few posts refering to similar situations but not once did I find an answer that worked. I will go into as much detail as I can about what caused it and what I've tried so far. 6 hours of failed attempts and I've made absolutely no progress whatsoever.

Before I Begin:

I have flashed different firmwares and ROMs to my G870A over the last couple years and occasionally ran into a soft brick here and there but never had any issues recovering with ODIN or a factory reset. I'm slightly familiar with the process.

-Installed and reinstalled Samsung drivers including the ones provided with Kies.

-Running ODIN as Administrator

-Using stock firmware files I have saved in the past that worked 100% of the time

-Factory reset/wiped cache

-Using the USB cable provided by Samsung

-Running Windows 7

-Service is AT&T

How I Bricked It:

It's been a year since I last did a major flash so I don't remember all the specifics but I believe I installed the 5.0 update from stock and applied the ROM file G870A_OC5_Stock_Rooted_ROM.zip and left it at that. It ran perfectly fine until I unrooted via SuperSU as I no longer needed root access. Immediately afterwards the app crashed and refused to open so I rebooted the phone. I did not apply any updates but I found myself stuck on the AT&T logo. I waited 15 minutes and nothing happend. Booted into recovery and factory reset/wiped the cache. Booted up again and waited 15 minutes at the logo. Nothing.

What I've Attempted So Far:

-Flashing G870AUCU1ANE4_G870AATT1ANE4_G870AUCU1ANE4_HOME.tar.md5 with Odin.

Results:

<OSM> Enter CS for MD5..

<OSM> Check MD5.. Do not unplug the cable..

<OSM> Please wait..

<OSM> Checking MD5 finished Sucessfully..

<OSM> Leave CS..

<OSM> All threads completed. (succeed 0 / failed 0)

<ID:0/006> Added!!

<ID:0/006> Odin engine v(ID:3.1101)..

<ID:0/006> File analysis..

<ID:0/006> SetupConnection..

<ID:0/006> Initialzation..

<ID:0/006> Get PIT for mapping..

<ID:0/006> Firmware update start..

<ID:0/006> SingleDownload.

<ID:0/006> aboot.mbn

<ID:0/006> NAND Write Start!!

<ID:0/006> FAIL! (Auth)

<ID:0/006>

<ID:0/006> Complete(Write) operation failed.

<OSM> All threads completed. (succeed 0 / failed 1)

-Flashing the original ROM I used to root the device: G870A_OC5_Stock_Rooted_ROM.zip. Same results.

-Flashing 2 other stock firmware files including one from sammobile. Same results.

-Tried a different version of Odin

-Tried to connect with adb from the command line. All commands result in error.

-Samsung Kies fails to detect the phone

Bootloader Screen When Attempting Flash:

ODIN MODE

PRODUCT NAME: SM-G870A

CURRENT BINARY: Samsung Official

SYSTEM STATUS: Custom

REACTIVATION LOCK(KK): OFF

KNOX WARRENTY VOID: 0x0

QUALCOMM SECUREBOOT: ENABLE (CSB)

RP SWREV: S1, T2, R1, A2, P1

SECURE DOWNLOAD : ENABLE

UDC START

SW REV CHECK FAIL : [aboot]Fused 2 > Binary 1

Recovery Screen Firmware Version:

Android system recovery <3e>

LRX21T.G870AUCU1BOC5

I would really appreciate any kind of help resolving this problem and retrieving access to my phone as its WAS in great condition. At this point I no longer care about tripping KNOX. Let me know if you need anymore information as I probably forgot something.