The first OS X El Capitan update arrived and I ran into the White Screen problem. Here is how I fixed it.
Apple released today the first update for OS X El Capitan. I was eager to install OS X 10.11.1 because of promised Mail bug fixes. As reported earlier this month, I encountered severe issues with Mail and Kernel_Task, rendering my two iMacs unusable due to CPU load.
I downloaded and started the OS X 10.11.1 update. After rebooting, the iMac would stay on a white screen and not continue to complete the load to the login screen. I tried reboot and again it hanged at the white screen. Then I did the usual reset of the SMC by unplugging the power cord. No change.
I then booted in the single user mode by holding down Command-S to run a check on the disk. I ran /sbin/fsck -fy on the command line, but there were no errors.
I took to Google and found the solution to my OS X 10.11.1 update problem - Safe Mode.
Holding down Shift while rebooting starts up OS X in safe mode. It is much slower as hardware acceleration of graphics is switched off. The short wait though paid off. My iMac continued to load to the log-in screen. After log-in, the OS X 10.11.1 update process continued and finished within a couple minutes. After restarting again, the iMac booted up without any problem with OS X 10.11.1 installed.
Apple says about Safe Mode, that it loads only required kernel extensions, prevents Startup Items and Login Items from opening automatically, disables user-installed fonts, deletes font caches and kernel cache and other system cache files. One of these things is responsible for hanging in a white screen during the upgrade to OS X 10.11.1.
OS X 10.11.1 as said has several fixes for Mail. So far Mail works fine, but last time the problems started to appear after a couple of hours. I will report tomorrow if OS X 10.11.1 indeed fixed the Kernel_task and high CPU usage issue.
Other fixes in OS X 10.11.1 include improved installer, fixes issues with Microsoft Office 2015, resolves Audio plug-in issues and most notably adds 150 new emoji characters. I personally don't know why one would need 150 additional emojis, I give it the middle finger after these upgrade woes, which actually is one of the new emojis.