Just in case that the problem is n your system and not the USB flash drive, you can try this solution..
Go to run->Type Regedit
Search into "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control \StorageDevicePolicies"
Double click to open Write Protected File and and change the value into " 0 " then click ok.
Insert your device and check it.
try to check this link..http://www.tekkianswer.com/2011/07/how-to-remove-write-protection-from-usb.html