Windows Boot goes through multiple phases which makes sure that your computer is not compromised. If you get a Windows Update Error Code 800704B8 - 0x3001A, it means that there is an extended error has occurred during the first boot phase.

You can choose to follow the below-mentioned suggestions to resolve it.

1]Disable or uninstall non-Microsoft antivirus applications.
2]Disconnect all unnecessary devices
3]and Perform a clean boot.
4]Check your Disk Space. It is possible that your computer might be in need of more space.

Here is another thing you can do, look for Setup Error log files which might be named as "xxxxxxx_APPRAISER_HumanReadable.xml, setuperr.log and setupact.log"

Search in those logs if there are instances of anything related to disk space. It may be similar to "DISKSPACEQUERY: Final disk space needed estimate". If that's the case, the update needs more space on your harddrive to complete the installation.