# 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: 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]

• Please add the output of hdiutil imageinfo filename.iso to your question, or post it to e.g. https://paste.ee and provide the link. – user3439894 Dec 23 '20 at 14:52