It's speculated this material is buried under a mountain, but we have satellite evidence of Iran digging and transporting something out, so nobody knows for sure. Iran also has nuclear facilities that were never inspected by the IAEA. It's possible there's undocumented nuclear material there.
And only removing the 60% grade material without removing the rest is completely silly. Iran has enough centrifuges to convert its 20% grade material into weapons grade in 1-2 months. And has enough 5% material to convert into weapons grade in 8 months to a year. Removing only the 500kgs of 60% grade material would not solve the issue, Iran would be back to where it was in a few months, and doing so would only kick the can down the road.
That's why all (several tons) of the nuclear material needs to be found and removed.
