ADB doesn't find N6 correctly

I have two N6s. One works fine for ADB, etc.

The other when attached to my PC via USB is recognized by Windows as Nexus 6. In Device Manager I can get it to come up as Nexus 6 and ADB Interface (both go away when I pull the USB out of computer)

I've tried toggling the USB Debugging and USB mode, revoking USB debugging authorizations, etc. and get various recognitions from Windows / Device manager.

I sometimes can see the N6 file system in Windows Explorer, When I can, I can drop files to the phone.

But when I connect via USB and try the ADB Devices command, it doesn't find the device. FASTBOOT Devices also doesn't work.

When I try to execute ADB commands on this phone only, they don't work. I tried to sideload an OTA and the app terminated unusually. (but I assume getting ADB Devices to work is the first priority).

This N6 has a locked bootloader and is not rooted, running Stock. At this time I just want to try to flash the July OTA security patch to it.

What am I doing wrong?