I am in a corporate environment which prevents write access to disc drives from userland unless you're using endpoint security software to encrypt the data before it gets written.
This still allows full read access to removable media however, but it seems the implementation for extracting the boot image requests write access?
I suspect the issue lies in attempting to open the device with write permissions, but hopefully this is a bug because I don't see why write permissions are necessary to extract the boot image.
Note I tried this on both a physical device and a virtual one.