If the drive does not have a physical lock, it is a Windows security feature that you need to disable with a registry key change:
You can remove write protection from your usb flash drive by adding/modifying a key like this:
In start button window search bar type regedit and hit enter to open the Registry Editor.
Then go to this key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\StorageDevicePolicies
If the registry key StorageDevicePolicies does not there yet, you will need to create it manually. To do that just click on the Control folder and under Edit select New and then Key -- name it StorageDevicePolicies, then add a new sting value and name it WriteProtect
Double click the key WriteProtect in the right window and set the value to 0 in the value data box and click OK
Restart the computer. Should be able to copy files to USB drives again.