Was having a similar problem awhile back.
I did not encounter any issues when first time upgrading my laptop, a Sony Vaio SVS 15126PG
running windows 8 pro with switchable nvidia/intel graphics, to 8.1 in November. While
doing a windows update on 25 Dec, got a total black screen and system hang. The update
appears to be an optional one - graphics adapter wddm1.2 graphics adapter wddm1.3 - intel
(r) hd graphics 4000 but not sure it's the cause of the problem. Had to roll back to
windows 8 factory setting as both refresh and system restore didn't work.
Having read through many postings on this site and taking recommended steps such as clean boot,
installing latest drivers etc, and COUNTLESS attempts after, still stuck with a black
screen while 'Getting devices ready x%' during the installation process and then reverting to
8 with several error codes at one time or another e.g. 0xc1900101 0x40019, 0xc1900101
0x4000D, 0xc1900101-0x30018.
I then came across a posting that suggested uninstalling and deleting all graphics drivers
but with NO REBOOT. The steps I took in 2 attempts are listed below.
First attempt (unsuccessful)
-- start by reverting back to windows 8 factory setting
-- done update from vender (sony in my case)
-- done all important updates from windows, leaving out all optional updates
-- uninstalled plus deleted both intel/nvidia graphics drivers but with NO REBOOT
-- downloaded 8.1 installer from window store and started installation process
-- upon restart, it got to 'Getting devices ready x%', failed again and reverted back to 8
Second attempt (successful)
-- uninstalled plus deleted both intel/nvidia graphics drivers but with NO REBOOT
-- started download 8.1 installer from window store and it went straight into installation
-- this time after restart, the installation process went right to the end with 8.1 successfully
installed
The above steps seemed to have fixed the problem for me (still don't know why it took 2
attempts), and I'm not sure it's helpful to anyone but hope Microsoft come up with real
solution soon.
Good luck.