I received this info in response to a ticket I submitted to HP support:
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Hello,
My name is Randall Guzman from HP Second Level, we received an escalation regarding issues with Windows update for the HP mt43 Thin Client.
I was able to replicate the issue on my lab and found that there is a KB needed in order to install the cumulative security updates for Win 10 IoT.
This KB is a pre-requirement for the following KB’s: KB4284880 & KB4338814.
The servicing stack update (SSU) (KB4132216) must be installed before installing the latest cumulative update (LCU) (KB4338814 & KB4284880).
The LCU will not be reported as applicable until the SSU is installed.
This is the download link for the KB4132216: http://www.catalog.update.microsoft.com/search.aspx?q=kb4132216
Please make sure to download the x64 package for Windows 10.
Let me know if you have any question for this case.
Regards,
Randall Guzman Mora
2LS – GSE PPS AMS
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I was also able to get one of the units to update using our WSUS server:
http://h10032.www1.hp.com/ctg/Manual/c05310155