Create a U-boot disk for Ubuntu with Mac OS

Download Ubuntu release

wget https://releases.ubuntu.com/focal/ubuntu-20.04.5-live-server-amd64.iso

Using hdiutil to convert ISO to dmg

 hdiutil convert -format UDRW  -o ~/Downloads/ubuntu-server.dmg  ~/Downloads/ubuntu-20.04.4-live-server-amd64.iso

Check the serial number of the USB flash drive in Mac OS

diskutil list

Umount USB disk

diskutil unmountDisk /dev/diskN 

Write the image to a USB flash drive

sudo dd if=~/Downloads/ubuntu-server.dmg of=/dev/rdiskN bs=1m

Eject the USB flash drive

diskutil eject /dev/diskN