Hi John,
Sorry for not clearly understanding your issue.
Do you mean you have a ISO image located in USB flash disk, and want to make some modification in that ISO image. So that you need to mount ISO image before modification, right?
If yes, we could use the following command line to mount it, it has readonly parameter to use.
DISM /Mount-image /imagefile:<path_to_Image_file> {/Index:<image_index> | /Name:<image_name>} /MountDir:<target_mount_directory> [/readonly] /[optimize]}
For more information, please refer to: Modify a Windows image using DISM
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Bests,
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