Are you absolutely sure that you haven't picked up a group policy to enforce write protection on USB keys?
This can be over-ridden by setting the following reg key:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\StorageDevicePolicies]
"WriteProtect"=dword:00000000
The key is Hexadecimal, not decimal. The value should be '0'. If it's currently enforcing write protection, the dword value will be Hexadecimal format, value '1'.