[One (M8) Q&A, Help & Troubleshooting] istrasci:
Preface

- I don't know what half of the dang rooting terms mean (My own fault before getting into all of this :crying:)

- Verizon HTC One M8

- Had Unlocked, Root, and S-OFF for 2 years

- Had some version of TWRP recovery

- Trying to get it back to stock recovery and "ROM" (correct term?)

- Connecting phone to OSX for adb/fastboot and could successfully run commands for both.

- fastboot getvar all output (with values I believe to be sensitive starred-out)

(bootloader) version: 0.5
(bootloader) version-bootloader: 3.19.0.0000
(bootloader) version-baseband: 1.12.20.1211
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main: 4.17.605.5
(bootloader) version-misc: PVT SHIP S-OFF
(bootloader) serialno: *************
(bootloader) imei: ***************
(bootloader) imei2: Not Support
(bootloader) meid: ***********
(bootloader) product: m8_wlv
(bootloader) platform: hTCBmsm8974
(bootloader) modelid: 0P6B20000
(bootloader) cidnum: VZW__001
(bootloader) battery-status: good
(bootloader) battery-voltage: 0mV
(bootloader) partition-layout: Generic
(bootloader) security: off
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: f2489bd1
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0
all: Done!
finished. total time: 0.017s

What I've tried

- I downloaded an RUU 0P6BIMG.zip from a link on some M8 thread in XDA; couldn't even tell you which one (but I make sure the posts I view are related to Verizon M8). My understanding is that an RUU is an all-in-one tool for stock: stock bootloader, recovery, and ROM. My understanding may or may not be correct.

- I connected my phone to the computer (again, OSX), and I could see my device listed with fastboot devices.

- Following Method 1 on this page, I rebooted into "RUU mode" (term?) with fastboot oem rebootRUU. This showed an all-black screen with a white "htc" in the center.

- Then I flashed the RUU I'd downloaded with fastboot flash zip 0P6BIMG.zip

- This started copying files / doing whatever. Took about 5 minutes and seemed to be working, but ultimately it ended with a "FAILURE: something something" (unfortunately, I didn't capture the exact output).

Where I'm at now

- I believe I'm still in "RUU mode" with failure indicators. It's still an all-black screen with the white "htc" in the center, but now each of the 4 corners of the screen also has a white "warning" symbol (triangle with exclamation mark).

- I can still see my device listed on my computer with fastboot devices, but cannot get any adb commands to work. fastboot getvar all gives the same result as it did prior.

- I attempted to re-flash the RUU file, but it again failed, only within a matter of seconds, not minutes.

- I downloaded an hboot.img (again, from a link on some XDA M8 post), and can successfully run fastboot boot hboot.img. Not sure what it actually does.

- I downloaded a new TWRP image file, twrp-3.0.2-0-m8.img from their site. I am able to successfully flash it with fastboot flash recovery twrp-3.0.2-0-m8.img. I assume this means it's "permanently" on the phone, not just some temporary measure.

- I'm also able to boot into it TWRP fastboot boot twrp-3.0.2-0-m8.img.

Now what?

- I cannot boot into the bootloader (white screen with options; I assume -- possibly incorrectly -- that this is hboot) using fastboot reboot-bootloader.

- Where do I go from here? I feel somewhat relieved that I can boot into TWRP, but not sure what to try now. From what I gather, the "RUU mode" with failure indicators stemmed from the failed RUU flash. So I thought to try to find another RUU file from a different source in case the original that I'd used was corrupted. Other than that, I'm pretty lost.

Please help!