USB Flash drive 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 the "Read Only" option.
For USB keys of the Transcend brand
You can try to make use of the JetFlash Online Recovery tool to repair Transcend USB keys (either by maintaining the data stored or by formatting the key).
Link: http://www.transcend.nl/Products/online_recovery_2.asp?dw=1
