1. Locate the USB disk you are using. My example (/dev/disk4) is shown below.
# diskutil list
...
/dev/disk4 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *61.5 GB disk4
1: DOS_FAT_32 USB64G 61.5 GB disk4s1
2. Erase and partition the USB disk.
# diskutil partitionDisk /dev/disk5 GPT "Free Space" x 0
3. Unmount the USB disk.
# unmountDisk /dev/disk4
5. Convert ISO image to DMG image.
# hdiutil convert -format UDRW -o ubuntu ubuntu-20.04.3-desktop-amd64.iso
6. Create the bootable USB disk.
# sudo dd if=ubuntu.dmg of=/dev/disk4 bs=1m
