Disk Utility won't allow me to Erase or Repair the drive, it will only let me verify it.
Clicking on Disk Partition, I get the following message:
This partition cannot be modified. This disk is not writable and can not be partitioned.
I used this thumb drive to transfer files from my mac to a PC, and it became write protected after a transfer from one to the other.
Best Answer
Before you do anything serious...
Rarely some memory cards/Pen Drives are having physical button/switch (yes a miniature mechanical switch) to make it write protect..
Check your pendrive to make sure you did not accidentally activate it by puling/pushing the USB.