'no signed device drivers' can be found on windows bootcamp installation

The message about 'no signed drivers' implies that the BC drivers may be corrupt. What is the Product Version in bootcamp.xml?

6.0.6133 does not support W8.1. From Install Windows 7 and earlier on your Mac using Boot Camp - Apple Support (and this also works on W8.1), you need Boot Camp Support Software 5.1.5640 . We can test replacing the $WinPEDriver$, AutoUnattend.xml and Bootcamp folder on the USB with what comes from 5.1.5640.

There are two sets of drivers. $WinPEDriver$ is the directory used to load drivers which may be needed during the install for hardware which are not natively available with Windows Installer. WinPE == Windows PreBoot Environment.

WinPE drivers are not installed when BC drivers are installed after Windows finishes installation. The post-installation drivers are in the Bootcamp -> Drivers folder. Some of them may have the same drivers. For example Broadcom BT and Intel GPU drivers.

You picked the worst driver possible for your test - AppleSSD.sys. It has a lot of issues and is unsigned in some versions of BC packages and cause a lot of grief.

In AutoUnattend.xml...

Notice the value of pass variable and the directory - $WinPEDriver$.

<settings pass="windowsPE">

<component name="Microsoft-Windows-Setup"processorArchitecture="amd64"publicKeyToken="31bf3856ad364e35"language="neutral"versionScope="nonSxS"xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<UserData>

<ProductKey>

<WillShowUI>Always</WillShowUI>

</ProductKey>

</UserData>

<UpgradeData>

<Upgrade>true</Upgrade>

<WillShowUI>Always</WillShowUI>

</UpgradeData>

</component>

<component name="Microsoft-Windows-PnpCustomizationsWinPE"processorArchitecture="amd64"publicKeyToken="31bf3856ad364e35"language="neutral"versionScope="nonSxS"xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<DriverPaths>

<PathAndCredentials wcm:keyValue="1" wcm:action="add">

<Path>$WinPEDriver$</Path>

</PathAndCredentials>

</DriverPaths>

</component>

</settings>

There is a "specialize" section. Notice the For loop which loops across all valid Windows driver letters looking for available files/drivers. (In theory if you added A and B drives, you can use a floppy drive, but it is no there right now).

<settings pass="specialize">

<component name="Microsoft-Windows-Deployment"processorArchitecture="amd64"publicKeyToken="31bf3856ad364e35"language="neutral"versionScope="nonSxS"xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<RunSynchronous>

<RunSynchronousCommand wcm:action="add">

<Order>1</Order>

<Path>cmd /c "FOR %i IN (Z Y X W V U T S R Q P O M N L K J I H G F E D C) DO (FOR /F "tokens=6" %t in ('vol %i:') do (IF /I %t NEQ "" (IF EXIST %i:\BootCamp\BootCamp.xml SETX AppsRoot %i:\ -m)))"</Path>

</RunSynchronousCommand>

</RunSynchronous>

</component>

</settings>

The oobeSystem (Out-of-Box Experience - https://technet.microsoft.com/en-us/library/cc766165(v=ws.10).aspx) pass has the BC driver installation, and disables all User Input...

<settings pass="oobeSystem">

<component name="Microsoft-Windows-Shell-Setup"processorArchitecture="amd64"publicKeyToken="31bf3856ad364e35"language="neutral"versionScope="nonSxS"xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<FirstLogonCommands>

<SynchronousCommand wcm:action="add">

<Description>BootCamp setup</Description>

<CommandLine>cmd /c %AppsRoot%\BootCamp\setup.exe</CommandLine>

<Order>1</Order>

<RequiresUserInput>false</RequiresUserInput>

</SynchronousCommand>

</FirstLogonCommands>

</component>

</settings>

The commands are fairly intuitive and you can see what they can do in the CMD window help.

Please ensure your backup OS X and Windows. Create a Windows System Restore point. Check Control Panel -> Boot Camp.