ADB Pull possible on boot loop? (Should have been unlocked but maybe not?) - Nexus 9 Q&A, Help & Troubleshooting

Hello everyone,
I've been googling this for the past couple days it's been driving me crazy. Stock 7 (I think), should have been unlocked but is showing locked.
1. My friend gave me the Nexus 9 which turned off one day, then wouldn't turn on again.
2. I plugged it in, and it turned on fine. Assumed bad usb port, he said keep it. I enabled developer options, then did "OEM [bootloader?] unlock" and "USB Debugging" toggles on (from memory, may not be right phrasing)
3. While trying to back up his data on it by ADB Pull, it would randomly stop. Using it, it would reboot unexpectedly.
4. So I went to Software Update.. bad idea in retrospect probably it rebooted during or something? It was fully charged and on AC power but I wasn't watching it.
5. I come back and it keeps bootlooping. Went into recovery, says "Device Locked" - Tried clearing cache, no difference. Tried sending bootloader unlock command with fastboot, says I have to delete all data to unlock.
The only lead that I have is that it might be possible to image TWRP to recovery, then boot into it so that I can get files off it (With OTG Flash drive or USB?) - I wanted to ask for help here first.
I did my best to search for this prior, but I've been out of the ROM game since 2012 cyanogenmod days.. Hope that it's possible to save my friend's data, and then also to flash a new ROM which will solve the reboot issue.. Thanks!

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Boot to recover, then when you get the Android mascot with the belly open, press ( I think vol up plus power) to get to a functional recovery with ADB support.

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gk1984 said:
Boot to recover, then when you get the Android mascot with the belly open, press ( I think vol up plus power) to get to a functional recovery with ADB support.
Click to expand...
Click to collapse
Thanks GK,
After going into recovery mode, there's the android mascot with the belly open as you describe. Then, I do (VolUp+Pwr) and it produces this screen below, which I'm guessing people call "fastboot". When I'm there, my Win10 PC makes the "USB item attached" noise, and I can use fastboot commands, but adb devices won't show anything. Do you have any ideas?
{
"lightbox_close": "Close",
"lightbox_next": "Next",
"lightbox_previous": "Previous",
"lightbox_error": "The requested content cannot be loaded. Please try again later.",
"lightbox_start_slideshow": "Start slideshow",
"lightbox_stop_slideshow": "Stop slideshow",
"lightbox_full_screen": "Full screen",
"lightbox_thumbnails": "Thumbnails",
"lightbox_download": "Download",
"lightbox_share": "Share",
"lightbox_zoom": "Zoom",
"lightbox_new_window": "New window",
"lightbox_toggle_sidebar": "Toggle sidebar"
}
I thought I could use the fastboot access I have to flash twrp, but I get the following error. I switched the "OEM Unlock" toggle, but i'm guessing that doesn't mean anything until I unlock the bootloader in recovery (which necessitates deleting all the files). I followed the directions on the twrp website which claim that root isn't needed, but I guess unlock is.
Code:
PS C:\Users\User Profile\Downloads\platform-tools_r30.0.1-windows\platform-tools> .\fastboot flash recovery twrp.img
Sending 'recovery' (13254 KB) OKAY [ 0.838s]
Writing 'recovery' (bootloader) Device State : Locked
FAILED (remote: 'Can not flash any images while device state is locked!')
fastboot: error: Command failed
Thanks again,
luciant

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luciant said:
Thanks GK,
After going into recovery mode, there's the android mascot with the belly open as you describe. Then, I do (VolUp+Pwr) and it produces this screen below, which I'm guessing people call "fastboot". When I'm there, my Win10 PC makes the "USB item attached" noise, and I can use fastboot commands, but adb devices won't show anything. Do you have any ideas?
I thought I could use the fastboot access I have to flash twrp, but I get the following error. I switched the "OEM Unlock" toggle, but i'm guessing that doesn't mean anything until I unlock the bootloader in recovery (which necessitates deleting all the files). I followed the directions on the twrp website which claim that root isn't needed, but I guess unlock is.
Thanks again,
luciant
Click to expand...
Click to collapse
You need to be in recovery for adb to work. The Android with belly open is recovery but you need to unlock it with a button combo. Fastboot won't work unless you unlocked the bootloader.
I may have the steps a little off for getting to recovery but I'm sure searching how to boot to recover for the Nexus 9 will find you the right steps. It's been a long time since I've used the stock recovery on this tablet.

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For future bootloopers: Well in the end, I wasn't able to figure out how to pull files using the stock HTC Nexus 9 recovery. Since it was locked, I wasn't able to install or execute TWRP .img , and unlocking it meant losing the files. It was weirdly looping even in Recovery & Fastboot. I tried every mode, and different drivers in the device manager, but ADB couldn't connect. Maybe it's not meant to for security purposes.
If you're bootlooping even in recovery, try the following:
I gave up on trying to recover the data, and decided to just unlock the bootloader. booted to Fastboot (Pwr+Vol Down on start up, select Recovery, Pwr+Vol Up, should say "Fastboot") Then I used fastboot oem unlock, which unlocked the bootloader and Deleted ALL DATA. Then, I could flash TWRP, using fastboot flash recovery twrp.img. Once I flashed TWRP, I booted to twrp recovery using Pwr+Vol Up at boot, wiped all partitions. Then I went to TWRP Advanced -> ADB sideload. Issued adb sideload ota.zip, and this gave me a clean stock install.
Thanks gk for your advice.