How to Extract the disk image from the USB thumb drive

$ mount
you should see the USB thumb drive mounted
/dev/disk3s1 on /Volumes/JIANG-USB

Before you extract the USB disk image, you should unmount the disk. Otherwise, it give you device busy message.
$ sudo umount /dev/disk3s1
For mac:
$ sudo diskutil unmount /dev/disk3s1

Then you could use dd to extract the USB disk image:
$ dd if=/dev/disk3s1 of=~/USB_Image.raw