in my computer right click on the USB drive and see if the Read only box is checked - if so remove the check and hit apply to remove the read only selection (if that does not work you may need to open a command prompt and use DOS commands to remove the atttribute.
This usually happen when the pendrive is near their broken state, but to be sure,
1. check whether there are physical lock on your pen drive, and switch them off
2. Do chkdsk X: /f (where the x: is your pen drive)
3. use this tools to format your pen drive (be caution that it will delete all your file in the pen drive, so be sure to backup them first) http://www.apacer.com/en/support/downloads/HS2.0_Utilit... (it will work for non apacer brand also)
4. if above still doesn't work, return it to your manufacture to replace it