ADB RSA Authorization Key dialog will not open

Question

so I'm trying to debug on my Nexus 6. I can't get the phone to open the RSA key auth. It works instantly on my friends macbook, with the same cable

• I have tried updating SDK.
• Installing and reinstalling the universal ADB driver; https://plus.google.com/103583939320326217147/posts/BQ5iYJEaaEH
• Checked that googles USB driver is installed and uptodate
• Rebooting both pc and phone several times.
• Killing and starting ADB from cmd.
• Switching between PTP and MTP modes
• And revoking USB debug auths on the phone...

Anyone have any ideas? I'm almost pulling my hair out. :(

Answer 1

1. Connect your Android device to computer using USB cable.
2. On your Android device go to Settings -> Developer options.
3. Find "Select USB Configuration" setting and set it to "MTP (Media Transfer Protocol)".
4. Wait for authorization prompt. It should appear within several seconds.

If it doesn't work, try "PTP (Picture Transfer Protocol)".

Answer 2

Short answer: go to C:\Users\<username>\.android and delete both adbkeyand adbkey.pub

Background: running stock LMY48I on Nexus 6, I could connect ADB, but it did not request authorization on the PC (Windows 10). The only way to get an auth prompt to come up was using Nexus Root Toolkit (wugfresh.com/nrt). It then asked for RSA key auth, and after being authorized, the device listed as connected and authorized (both using the NRT tools and ADB command line). As soon as NRT is closed, the device goes back to unauthorized, per command line ADB, and it didn't ask for auth again. Only deleting the key as per above enabled command line ADB without NRT to connect to the device.