Disc drive question

I have a 5 yr old Snapper rear engine 12.5 28" deck rider. I'm putting a new drive disc on it and I'm not sure what side the thrust washer (round donut shaped gasket) goes on. Inside disc drive or outside? It's on the outside and tractor is jammed in gear. It won't roll in any gear with clutch in or out. Im not sure what the problem is. Any ideas? Thanks