hdiutil convert -format UDRW not truly editable

Trying to add a single file to an existing ISO I ran into a bit of hiccup I can't see through. Here it is: Having never done it before but encouraged by reading the advice from this question: https://apple.stackexchange.com/questions/228116/add-delete-modify-files-within-a-disk-image-dmg I ran: hdiutil convert -format UDRW -o EditableImage.dmg givenImage.ISO and although it created a DMG that was was "mount-able" - I was not able to edit it in any way. Trying to run hdiutil resize -size 400m EditableImage.dmg gave me a failure with Resource temporarily unavailable. (I rebooted just to be sure there was nothing using it) running hdiutil imageinfo EditableImage.dmg indicated it was of Format: UDRW but I couldn't actually write to it.. and this led me to asking here the question: what am I doing wrong? [Runnning Catalina on a less than 1 yr old Mac with all updates]