I have a Surface Pro 4, 128GB, i5 CPU. It came with Windows 10 Pro x64 pre-installed. All updates prior to the AU have installed without any hitches.
Windows Update hasn't offered me the AU on the Surface, nor on my dual boot desktop.
Prior to the upgrade my desktop had version 1511 build 10586.494 installed , plus the "Insider" version 1607 build 14393.10. Each of these systems is installed on a separate, dedicated SSD.
I used the "Learn more" option to get the update on the desktop. That system updated without any drama. I didn't disconnect any of the other five physical drives, nor did I disable the Vipre antivirus/security app.
I then attempted to update the Surface using the same method. It failed to install around "Working on updates 95%" and reverted to version 1511. But didn't give any error message. I tried again, with the same failure. Today I downloaded the MCT and installed the AU on a flash drive. Using that to attempt to install the update also crashed at 95%, but it did throw up an error message:
"We couldn't install Windows 10
We've set your PC back to the way it was right before you started installing Windows 10.
OxC1900101 - Ox4001E
The installation failed in the SECOND_BOOT phase with an error during PRE_OOBE operation"
My searches for that error message didn't find anything specific to either Surface Pro or the Anniversary Update. Not even any recent references. I did find suggestions that that having an external drive connected and AV running had caused problems in the past for some users.
So I disconnected my external drive, uninstalled my AV (not just disabled it) disabled Malwarebytes anti-malware and anti-exploit. Copied the update files from flash drive to the microSD card, (so there would be no connected USB devices) and ran setup.exe from there.
I now have version 1607 build 1439.0 according to winver. The cumulative update to 14393.10 is waitjng for a restart as I type. Now build 14393.10. Yay.
Posting in case there is someone else with similar problems.