How can I force format read only usb drive in OSx El Capitan?

I have a crashed laptop and used the following method to make a bootable usb drive (32 GB).

hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso

diskutil unmountDisk /dev/diskN

sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m

It did work and made a read only usb that acted as if it were a CD. I would like to now undo that (in osx) but disk utility doesn't seem to be able to. When I click "Erase" it simply fails with

"Erase process has failed"

And details reveal

Name invalid.

Operation failed...

Is there a way I can force format this USB drive (to either FAT 32 or OSX Extended) in OSx El Capitan?