When I highlighted the Sno Leopard partition and clicked on "Erase", that partition dismounted. Then I got the POSIX error message. Using a menu command in DU, I could remount that partition. And I tried to erase it again with the same result.
As I reported, I then deleted that partition and recreated it using DU. I cloned Lion to it from the computer's HD using CCC. Just to test, I then used DU to erase that partition. This time DU succeeded in erasing it. So I again cloned Lion from the computer's HD to that partition.
Whatever the original glitch was, it was not DU's inability to unmount the Sno Leopard partition.