Thank you for the tip :
Removing write-protection using Diskpart
With your USB drive plugged in, launch a command prompt. Do this by searching for cmd.exe in the Start menu (or Start screen in Windows 8). In Windows XP, click Start then Run, and type cmd.exe in the box.
NOTE: you may need to run Cmd.exe with administrator privileges if you see an "access is denied" message.
To do that, right-click on the shortcut and choose Run as administrator. In Windows 8, simply choose Command prompt (admin).
Now, type the following, pressing Enter after each command:
select disk x (where x is the number of your USB flash drive - use the capacity to work out which one it is)
attributes disk clear readonly
create partition primary
format fs=fat32 (you can swap fat32 for ntfs if you only need to use the drive with Windows computers)