Hi, I had exactly the same issue on my Galaxy S2 and i found the easiest way to be able to drag n drop files was to plug in your galaxy via USB and let your PC install the drivers automatically, then go to>Control Panel>All Control Panel Items>Devices and Printers then right click your phone icon>Browse files then click on the phone model number. Once you have accessed the phone this way it seems to let you drag n drop files from your PC to phone and vice versa without any issues I really hope this works for everyone else as it did for me.
"I feel sorry for people who don't drink. When they wake up in the morning, that's as good as they're going to feel all day." ~Frank Sinatra