Mac Sierra won't boot - SmartBattery: finished polling type 4 I did an update on my Macbook Retina 13" to the most recent version of Sierra OS X. The computer never gets past the Apple logo (black screen with a full progress bar). 

I booted into Verbose Mode (command + v) and see that it is stuck at the following point:

SmartBattery: finished polling type 4

Every now and then it will add another line of text (same text) but it never moves beyond that point.

Any advice please! It is a real pain to keep restoring from my Time Machine backup!
Had the same issue and found the solution here: https://discussions.apple.com/thread/7787207?start=0&amp;tstart=0

Quotes:


  Now that I look a bit more closely, the issue may be a corrupt .kext
  file as I get two busy timeout [x] (60s)  kextd wait(1): messages with
  corrupt garbage characters listed just at the start of the infinite
  SmartBattery messages.
  
  Not convinced the battery is really the issue as the machine will boot
  quite happily when using another boot disk or USB drive and the
  SmartBattery checks in those boot logs are really no different. To me
  it looks like the boot process has hung at the kextd and the endless
  SmartBattery messages are the OS just passing the time waiting for a
  response to the kextd daemon its never going to get.
  
  Your (and my) problem is because auf a KEXT file which is not
  compatible to 10.12.2 - the software isn't clever enough to ignore
  this, so you have to do it yourself. There are some ways. I made it
  this way:
  Start into Recovery HD, Open Terminal, write down: 


mkdir /Volumes/"Macintosh HD"/kextbackup
cd /Volumes/"Macintosh HD"/System/Library/Extensions



  look for .kext files which are not part of the normal system


mv KEXTNAME.kext /Volumes/"Macintosh HD"/kextbackup/



  I removed the following and after this the system booted again.


Belcarra.USBLAN_netpart.kext
Belcarra.USBLAN_usbpart.kext
HuaweiDataCardDriver.kext
JMicronATA.kext
RemoteControl.USBLAN_usbpart.kext
UsbEthernetGadget.kext
USBExpressCardCantWake_Huawei.kext
vecLib.kext



  Of course everybody
  has his own not compatible kext files in this folder. Actually you
  can't do anything wrong by just moving them to another place... But I
  would recommend you not to move any of the APPLE*.kext files and the
  IO*.kext files. Other files with not that normal names, I would move
  and try to start.... At the moment my Sound is not working.. So I
  moved one kext too much, but anyway... I can write this.. in Safari on
  my Macbook Pro Retina....


EDIT: This worked for me. Note that I also had no sound but got it back after moving back "vecLib.kext" (by again returning to recovery mode and moving it back from the kextbackup-folder), so try to keep that in place from the beginning :)