Unable to format sandisk ultra 16gb usb3.0 pendrive in mac
Your USB stick may be defective.


  How do I fix a Write Protect error?
  
  Write protection errors occur when a flash drive detects a potential
  fault within itself. The drive will go into write-protected mode to
  prevent data loss. There is no method to fix this. To verify the issue
  is related to the flash drive and not your computer we recommend that
  you try plugging the USB drive into another port on your computer. You
  should also try the drive on another computer if possible.


Source: https://kb.sandisk.com/app/answers/detail/a_id/8656/~/write-protect-error-on-usb-flash-drives