Here Today, Gone Tonight - S5-E17
Continuity mistake: Danno boards the helicopter without his briefcase, but arrives to conduct the interview carrying the case and tape recorder. He takes the copter back home again with nothing in hand, getting into his car empty handed as well. But he enters McGarrett's office carrying the briefcase again.
Answer: He was fired. He never really appreciated his character. He felt Kono was portrayed as a big dumb Hawaiian and that the stereotype was racist. He also felt underutilized. He was fired after a heated argument with the show's publicist regarding his character. It seems there's not much details given regarding the incident or the firing, so it's seems possible he upset the show's producers as well.
Bishop73
No he said something derogatory about one of the Jewish producers that's why he was fired.