Good to see that you get it work.
It seem that there is differences on USB devices depend on is it memory stick or external disk.
Most of the time I use WU to update Insider previews, so I use USB -installer
only for clean install's, but I start testing this issue just for fun on rainy Sunday.
That thread is in Insider forum, but "May Update 1903 is out" whole discussion is about released v1903, not previews.
Hi Pauli O,
It looks like they just block the devices that show the RMB (Removable Media Bit).
The upgrade works fine from devices that do not have the Removable bit on.
Anyway, it seems that the block was added in the installation media files created on 4/1/2019 4:59PM (Build 18362.30) .
MCT is now downloading a new ISO with files created on 6/11/2019 Build 18362.175, this seems to have removed the block for the USB removable from the installer, the upgrade from the USB stick proceeds normally.