EXCELLENT and highly technical posts appear to share some helpful early information on this issue. Resolution of issue may be associated with Profile paths or directory structures that the Windows Update process needs further adjustment for. As a number of WIN8 users have reported issues, WIN8 users should carefully monitor developments for possible replacement update in future.
QUOTE: We had already updated a Windows 8.1 x64 machine with KB2919442 and was starting to install Windows8.1-KB2919355-x64.msu. Firstly it was hanging for ages at the “Installing” phase with no progress… So why was D:\Users\Default folder missing? This is because the User Profiles has been redirected using a registry key, instead of unattend.xml – Refer to “Relocation of the Users directory and the ProgramData directory to a drive other than the drive that contains the Windows directory” at http://support.microsoft.com/kb/949977 So to fix the machine we ran: robocopy C:\Users\Default\ D:\Users\Default\ *.* /s To prevent these type of issues in the future however it is best to keep ProfilesDirectory on the system volume (i.e. C:\ ) This issue may not have occurred if the unattend settings had been used to put the profile on D:\ After creating a copy of default profile under D:\Users\Default we re-ran Windows8.1-KB2919355-x64.msu and updated installed without an issue