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Macbook hangs after installing OS X 10.11.1 (31 Oct 2015)

In more than two years of owning a Macbook, I never faced a single issue upgrading the OS. Until today when, after installing the OS X 10.11.1, the Macbook would just hang after rebooting.

More precisely, the progress bar on the boot screen would stop at roughly two thirds of the way. So today, finally, I had to figure out what kind of diagnosis and recovery options OS X actually provides.

The two most important tools were:

⌘+V during reboot: Boot in verbose mode, showing the boot messages instead of the Apple logo and progress bar. I noticed that the boot process hung after a message saying pci pause: SDXC.
⌘+R during reboot: Boots into recovery mode, which offers tools for disk and networking diagnosis, as well as the option to open a terminal as a superuser.

Things I tried:

Reset the System Management Controller (SMC)
Reset the NVRAM - this actually changed the behavior; the progress bar would now fill up to 100%, but then nothing happened anymore. When booting in verbose mode (showing all the boot messages), startup continued past the pci pause message, but stalled later in what looked like a loop involving messages liked IO80211AWDLPeerManager::setAwdlOperatingMode Setting the AWDL operation mode from SUSPENDED to AUTO and IO80211AWDLPeerManager::setAwdlAutoMode Resuming AWDL. (See also http://superuser.com/questions/917220/macbook-can-not-boot-os-x-yosemite-as-it-hangs for a description of similar symptoms.)
Repair my SSD in recovery mode - this found a number of issues on the disk and repaired them, but did not fix the hang-up issue.
Fix permissions using the recovery mode resetpassword utility
Boot info safe mode - this did not work at all, as the hang-up apparently affected safe mode as well.

I was dangerously close to reinstalling the OS now, with only two aces left up my sleeves: Single-user mode and kernel extensions.

I had seen references to startup issues related to kernel extensions in some articles, but knew very little about them. The first thing I tried was to temporarily disable kernel extension signing (nvram boot-args=kext-dev-mode=1), with no discernible effect. Fortunately, at this point I came across Justin Silver's blog article "OS X El Capitan 10.11.1 Hanging on Boot" in which he describes a startup issue which looks a lot like mine. His excellent summary contains instructions on disabling kernel extensions using recovery mode - which helped me solve my problem!

I booted into recovery mode, opened a terminal window, and then:

  $ mount -rw /
  $ cd /Volumes/Macintosh\ HD
  $ cd Library/Extensions
  $ ls
ACS6x.kext                   HighPointIOP.kext
ATTOCelerityFC8.kext         HighPointRR.kext
ATTOExpressSASHBA2.kext      PromiseSTEX.kext
ATTOExpressSASRAID2.kext     SoftRAID.kext
AX88179_178A.kext            hp_io_enabler_compound.kext
ArcMSR.kext                  hp_io_printerclassdriver_enabler.kext
CalDigitHDProDrv.kext

Following Justin's advice, I moved all kernel extensions out of /Library/Extensions to disable them temporarily, and then rebooted. Lo and behold, the login screen appeared! After a few such rounds of moving kernel extensions back to /Library/Extensions and rebooting, the root cause of my startup issues turned out to be those HP printer extensions.

Phew.

The HP kernel extensions had been on my system for quite some time, of course, and had never caused any problems like this before. So there are still open questions about what exactly had happened here. To be solved on a rainy day.