[Nexus 6 Q&A, Help & Troubleshooting] fromthebayou:
So I've been figthing this battle for the last couple of days... and no matter what drivers I installed (and trust me I've installed everything from the latest google drivers to Khoush's Universal drivers) I could never for the life of me get the Nexus 6 to show up during recovery - and hence couldn't sideload the 5.1 OTA. Mind you I'm completely stock - boatloader, recovery, kernel and all....

I Knew it was a driver issue - so I went back to the drawing board. Here's what I did and you should try.

Step 1 : Assuming you have SDK installed/Minimal ADB fastboot installed , download the latest usb drivers.

https://dl-ssl.google.com//android/repository/latest_usb_driver_windows.zip

Unzip and remember location.

Step 2 : Unplug your phone and Use usbdeview (google USBdeview) to uninstall all existing devices that say something like

nexus 6
android
google

Step 3 : With USB debugging enabled and MTP on , plug your phone in... If it installs drivers - dont worry. Go to device manager - look for anything that says Andoid/ADB composite device or MPT/Nexus. Right click and select Uninstall. Check Delete driver software for this device and press OK. You may need a restart, if windows tells you to. Either way unplug your phone.

Step 4: Plug your phone in. Let it try to figure out devices/drivers. Then go to Device manager. The Nexus should have an exclamation on it. Right click and go to properties-->Details. Select Hardware Id's from the dropdown. There should be one or two. Either one is valid. Now open the USB drivers folder you unzipped earlier and open the android_winusb.inf file with notepad (usually double click should open it).

If you're on a 32 bit operating system - scroll the [Google.NTx86] section.

If you're on a 64bit OS (like most on Windows 7/8/8.1) scroll to the [Google.NTamd64] section.

If you don't know which OS you are on - please wait for the OTA to arrive on your phone because this guide( and other guides) maybe beyond you :).

Scroll down to the Google Nexus (Generic) section for your OS type. It should say something like

;Google Nexus (generic)
%SingleBootLoaderInterface% = USB_Install, USB\VID_18D1&PID_4EE0
%CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4EE2&MI_01
....

Now from device manager copy the hardware id for your device. Say its

USB\VID_18D1&PID_4EE7&MI_01

If your Hardware Id (atleast one) isnt listed in the CompositeAdbInterface list on the driver file then insert a new line. Like below.

;Google Nexus (generic)
%SingleBootLoaderInterface% = USB_Install, USB\VID_18D1&PID_4EE0
%CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4EE2&MI_01
....
%CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_4EE7&MI_01 <----New line

Save the driver file. Now open device manager again and Right Click the nexus device... Update Device Driver --> Browse my computer.. --> Specify the newly modifed USB file.--> press Next to completion. If you are prompted saying device is unsigned... ignore it.

Step 5 : Open command prompt /cmd (Start-->search for cmd). Browse to the ADB location. Then type

adb devices

It should show you the a device. (You may have to allow adb usage on your phone from that specific computer - should popup on your phone when connected). Next type

adb reboot bootloader

Your phone will automatically reboot into the bootloader.If you see a device listed under Android Bootloader Interface of If you see a new nexus device in your device manager then Uninstall the device while deleting driver. If you dont see a device - use USBdebiew to uninstall devices. Right Click on the computer name in device manager and click scan for hardware changes. You now have a device showing up with Exclamation. Now add the hardware ID for this device to the usb file. Make sure you add it under CompositeAdbInterface and not SingleBootLoaderInterface. If your device is listed under SingleBootLoaderInterface - Then use a semi-colon ( ; ) at the beggining of the line to comment it out... and then add the Hardware Id to the CompositeAdbInterface. Save and update driver.

Now choose recovery from the options. At this point unplug your phone. Now hold the Power button on your phone for about 5 seconds and then press Volume UP. It should take you to the recovery options screen. Use your volume button to move to "apply update from ADB" and press Power Button to select. It will take you to another screen saying its ready for adb sideloading.

Now Plug your phone in. If you see a new nexus device in your device manager then Uninstall the device while deleting driver. If you dont see a device - use USBdebiew to uninstall devices. You now have a device showing up with Exclamation. Now add the hardware ID for this device to the usb file. Make sure you add it under CompositeAdbInterface and not SingleBootLoaderInterface. If your device is listed under SingleBootLoaderInterface - Then use a semi-colon ( ; ) at the beginning of the line to comment it out... and then add the Hardware Id to the CompositeAdbInterface. Save and update driver.

Go to the command shell and look for adb devices again. It should now list your Device.

Sorry about providing links as code : Dont have enough post apparently to directly link stuff.

Happy sideloading!!!