6_1 is a ribbon knot (drawings by Yoko Mizuma):
has two slice disks, by Scott Carter
Scott Carter notes that 6_1 bounds two distinct slice disks. He says: "this was spoken of in Fox's Example 10, 11, and 12 in a Quick Trip through Knot Theory
... BTW, the cover of Carter and Saito's Knotted Surfaces and Their Diagrams
contains an illustration of such a slice disk". A picture is on the right.