The windows update 'Upgrade to Windows 10 Enterprise, version 1511, 10586' fails with 'Windows can't be installed because this PC has an unsupported disk layout for UEFI firmware'.

This PC (laptop) is running Windows-To-Go (Windows 10) from a an external USB SSD. It has been running this way for a few months now without any issues. The 'original' system, i.e. the internal SSD of the laptop is running Windows 8.1 Enterprise and I had created the Windows-To-Go installation from there.

My Disk Management looks like this:
Disk0 (offline when using Windows-To-Go): 300MB (Recovery Partition), 100MB (EFI System Partition), 97GB (Primary Partition - Windows 8.1 Enterprise), 140GB (Primary Partition)
Disk1 (online = USB SSD): 350MB FAT32 (System, Active, Primary Partition), 238GB NTFS (Boot, Page File, Crash Dump, Primary Partition)

Any ideas how I can get the update version 1511, 10586 to install?