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In most of the USB based flash/pen drives, write protection is provided via the small notch. Look for it in sides of your drive. It’s even marked with lock/unlock symbol. Move/slide the notch to unlock position (means releasing write protection) before you perform any file operations in your drive such as copy/paste/format etc. After that you can slide back the notch to lock position safely.

But some of the pen drives don’t provide hardware lock/notch for write protection. Instead they back it up with by means of some software/tool program. From there you can change the write protection lock/unlock option. Refer to your pen drive’s manual for more details in this regard as different pendrive have diferent ways to to unlock them or otherwise you can also take help of some tools to unlock them.