I received a reply from HP Support today. They have found the problem and have a workaround:
The Fix for unmountable Boot Device is:
1.Log in administrator account, disable UWF 2. Change the value of the below registry from ‘0’ to ´3´.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Ramdrive\start
"
Problem will be definitely solved in the new win iot 10 release thats coming in April.
