[Galaxy Note 4 Q&A, Help & Troubleshooting] ren101099:
This question may contain some irrelevant information, but I wanted to give as much detail as possible. I can tell you basically everything I did to the phone from the factory state before I ran into the problem with not being able to boot. If it is too long to read, you could skim it and skip to "(now we are getting to the horrific part)" below.

I rooted my Samsung Galaxy Note 4 SM-N910P (Android 6.0.1) using the method described here:

https://xdaforums.com/t/sm-n910-cf-auto-root.2897428/

It is not my main phone. It is a 2nd, used phone which I purchased recently, and it was apparently in factory state or factory-reset state when I removed it from the envelope. It had Android 6.0.1 on it. The phone does not have any SIM card in it, nor does it need one, as I was planning on using it as a tablet connected to WiFi. I rooted it. This was my first time rooting a phone, but I didn't have any problems. Everything was OK, root successful 👍

I installed the following "SCR" app because I wanted to screen record with internal audio and simultaneously hear the internal audio through earphones (which was the reason I rooted the phone):

https://xdaforums.com/t/app-4-0-root-scr-screen-recorder.2422061/

But it did not work. Even though it was advertised as working with Android 6, the app would immediately crash after pressing the record button. It gave the SR135 error mentioned here:

https://xdaforums.com/t/app-4-0-root-scr-screen-recorder.2422061/page-45#post-67918177

The SCR app did give a warning when I enabled internal audio from the settings menu, but I think that this is (probably?) not the source of my boot problems, because I had since rebooted the phone several times with no trouble. I don't think the internal audio plugin even installed; it seemed to just silently fail to initiate the installation. It wasn't until I installed Titanium Backup that I had trouble booting. I had stopped attempting to use SCR, and was planning on uninstalling it.

The next thing I tried was installing some APKs for the Samsung Game Launcher, because supposedly that app allows recording of internal audio. I got the APKs following these links:

https://xdaforums.com/t/game-launcher-for-note-4-without-root.3437901/

These earlier versions of the Samsung game APKs were not suitable because they did not allow me to add arbitrary apps to the launch menu (so that I could screen record). Something about a requirement to request to register the app as a game, and it would not go through. So I tried some later versions (the latest that I could find on APK mirror that were compatible with Android 6), and those did allow me to add arbitrary apps, but I had problems with these later versions crashing before I could actually launch anything.

So, following my inspiration in my quest to screen record with internal audio while also listening to internal audio, namely, this detailed reddit post,

https://www.reddit.com/r/AndroidQuestions/comments/81vhvm

I then proceeded to install RecMe screen recorder and its internal audio plugin (the latter requiring root, which I had):

RecMe Screen Recorder - Apps on Google Play

Screen Recording - No time limit, HD, with Audio, no Root required

play.google.com

Internal Audio Plugin (ROOT) - Apps on Google Play

Internal Audio Plugin app for mobZapp (Screen Stream Mirroring, RecMe, ...)

play.google.com

This was an improvement over Mobizen for Samsung, which I had tried on my other, unrooted phone, and which could record internal audio on Samsung devices but which could not also simultaneous allow the internal audio to play on earphones/speakers (you hear only silence when recording, but the audio is in the recorded MP4). RecMe with the plugin could actually play internal audio while also recording the internal audio, although the sound quality in the earbuds was poor, and it needed more testing.

I rebooted successfully after installing RecMe and the plugin, so I think it (probably?) did not cause my boot problems.

I was planning on doing some more testing with RecMe after installing AFWall+ to block RecMe from accessing the Internet for security purposes, but I never got to installing AFWall+, because this happened (now we are getting to the horrific part):

So this Note 4 phone, SM-N910P (Sprint), every time you boot it, it tries to go through this Hands Free automatic activation process to connect to the Sprint network that takes a long time (I think 5 retries, taking like 60+ seconds each). This isn't going to work because there's no SIM card (as I said, I'm using it as a tablet, I don't need cell service). So I go to the App Manager -> Show System Apps and then stop the Samsung DM Service. This stops the annoying automatic activation attempts, but I have to do it every time I boot.

So, to avoid doing this after every single boot, I followed the instructions here to first back up, and then uninstall, Samsung DM Service:

How to Delete & Manage Unused Apps on Android: 2 Easy Ways

This wikiHow teaches how to disable or remove apps from your Android device that cannot normally be uninstalled, which may require root access for your device. Open your Android's Settings. If your device is not rooted you'll only be able...

www.wikihow.com

Per the instructions, this required installing Titanium Backup (root needed):

Titanium Backup (root needed) - Apps on Google Play

EXTREMELY powerful tool. Backup ALL apps, Market links, remove bloatware & MORE!

play.google.com

When I installed the app, I granted root or superuser or whatever it's called to it, and it prompted me to also install "Titanium Backup Add-on," which I did.

Titanium Backup Technical FAQ | Titanium Track

www.titaniumtrack.com

Then I used Titanium Backup to (first) back up Samsung DM Service, and (second) uninstall Samsung DM Service. After that, I got some messages that said "Unfortunately the process com.samsung.syncservice has stopped." OK, I'll deal with that later, I thought. I proceeded to reboot the device, because I wanted to test and see if the annoying Hands Free automatic activation thing would be gone.

Now, when I try to boot the phone, it briefly says "installing system updates" (which doesn't make any sense because it should already be on the latest possible security patch [I checked] for Android 6.0.1 for this past-end-of-support-lifetime Note 4 phone — I forgot to mention earlier that I updated the Android security patch level before I did the root, and the Play store updated after I did the root [and I successfully rebooted — no boot problems until Titanium Backup]).

And then, no matter how many times I try, the phone always just boots to Android Recovery. It says samsung/trltespr/trltespr 6.0.1/MMB29M/N910PVPS4DQF1 at the top, and at the bottom it says "dm-verity verification failed". It does NOT explicitly say anything about failed "drk" which I saw mentioned in some Google search results.

I tried repeating the root steps which had earlier allowed me to successfully root the phone:

https://xdaforums.com/t/sm-n910-cf-auto-root.2897428/

But no change. Same thing. Android Recovery. Tried reboot to system, to boot-loader, tried Wipe Cache, nothing works. Always boots into Android Recovery.

1) Can I fix this without doing factory reset?

2) Will factory reset fix this?

3) What did I do to provoke this? (so that I may avoid doing it again) It was caused by installing Titanium Backup? I thought this was a safe app? It has almost 400K reviews; I thought it was one of the most popular apps that require root? Or was it caused by uninstalling Samsung DM Service? Really? I did not think it was a super essential service; I had force-stopped it many times without any problems. (If so, in that case, how do I get rid of the annoying Hands Free Activation thing?)

4) Is it possible to access the filesystem (though USB?) to retrieve a screen shot? (The phone is not encrypted.) It's not a super big deal, but I made another Google account for this 2nd phone, and instead of memorizing the password, I took a screenshot of the random characters I had typed. I planned to learn it later. I know that was foolish, I guess, but I didn't think anything bad would happen. I had already successfully rooted the phone, and I didn't think I was doing anything super risky at this point. Literally all I did was install a few apps including Titanium Backup. And I uninstalled Samsung DM Service. Now I can't boot the phone. What is this horror?

[I am not an Android expert. I have used adb a few times, following other people's instructions. I don't have all the command memorized, I am not an expert, but I've used adb on my Oculus Quest (to change resolution) and also on my main phone that is unrooted (not on this rooted, boot-troubled one) in the course of installing Island on that other phone (it was never installed on this boot-troubled one). I don't even know if it's possible to use adb on a phone that cannot boot. But if you tell me adb commands I can paste them into the command window, assuming I am able to connect to an unbootable phone...]

If anyone can help me escape this horror, I will be forever grateful....