I tried marking mine active multiple times, and as you can see, it is not currently active. All I got was the message below. I also just noticed the USB drive is showing as GPT .. I did not do anything I know of for that to happen.
Maybe I am missing something but all I do is mount the .iso and copy the files to the NTFS partition. No DISM operations necessary..
I know, NTFS does not have the 4GiB file size limit.
What is in the FAT32 partition ? Are you booting from it ? How was it prepared ? How did the disk convert to GPT ?
Here are my 2 ISO Files, the first is the one downloaded from MS Insider with the install.wim
The second is with the install.esd,
If I mount the one with the .esd I can copy all the files to a FAT32 USB Flash drive with the Partition set Active.
That can boot from Both Legacy BIOS or UEFI machines.