Plug-in USB drive and open Disk Utility.
Create a partition on the USB Key or erase it completely.
Open Terminal.
Type diskutil list and locate the location or name of your USB Drive (/dev/disk1 for example).
Type sudo dd if=(drag ISO file to automatically fill the path) of=(usb key path) bs=1m
For example,
sudo dd if=/Caldera-Debian-2.1.iso of=/dev/rdisk1 bs=1m
if=input file
of=output file
bs=block size
Note: Change the device from /dev/disk1 to /dev/rdisk1 to copy faster.