cp command to make bootable iso image usb

Devices in Linux are files. /dev/sda or /dev/sdb are files as well. So what you do is replace the /dev/sdX file with the dot-ISO file, which you can then mount and use as a device.

What sync does, is it forces the changed blocks on the virtual device (/dev/sda file) to be updated on the actual disk.

A possibly better way of doing it is with dd. which does byte copy. the syntax would be

dd if=debian.iso of=/dev/sdX

and the command will need to be run as root. dd allows you to have more control if you want it. Check out it's man page here or Debian instructions on how to create a bootable USB stick here.

(Where sdX maybe the full name of the drive, with the number, for instance /dev/sdc1.)