I've seen many users here at WPCentral being affected by the endless spinning gears/cogs bug when they try to reset or update their phone and its nothing less than a nightmare.
Luckily the solution has arrived and its quite an easy fix. Nokia has updated its Nokia Software Updater to version 4.1.0 which has added an option called "My device does not boot up". So here's the procedure how to fix your phone plagued by endless spinning gears bug. I'll keep it short:
2. Run NSU and this window will pop up. Then click "My device does not boot up"
3. Now connect the phone that has been stuck at spinning gears via USB cable (make sure its fully charged) and click continue.
4. After connecting the phone, this will show up (refer screenshot). Now press and hold the power and the volume down button until the phone vibrates. Your PC will detect a new hardware and will install the required drivers if needed. Then click Ok.
5. NSU will take over from here and will reflash your phone with a hefty 1.2GB of firmware download from its server. Let it finish and then you're done.
Hopefully this will resolve the issue of users being troubled by this bug and will rejuvenate their phone back to life. Cheers.