dd is a command-line utility for Unix and Unix-like operating systems whose primary purpose is to convert and copy files.
dd is also used to make a usb flash drive bootable using an image (.iso, etc).
To make the usb stick usable again you must recreate the partition using a
Master Boot Record, can be easily achieved with Disk Utility:
Now the usb stick will be detected/usable on any machine (Windwos/Unix).