USB stick locked - Unable to write
In the case that you cannot write or save files on your USB drive, it is probably locked in read-only.
Read-only lock
As a first step, verify that the USB does not have a small switch (normally hold or locked written on it) disabling write mode. Unlock it and try to write.
File System
Check if directory in which you are trying to write is not read-only protected by the system. Right-click on the folder, and then uncheck Read Only.
Format the USB key
As a last resort, you can always opt for formatting the USB key.