A109 Captain reported on preflight and engine start; failure to release rotor break resulted in burned brake with smoke.

Narrative

Due to the windy conditions locally for the last three or four days the helicopter blades were tied down and rotor brake applied. As we prepared for the flight and removed the tie downs; one of the medical crew removed the tie downs bag that was tied to the rotor brake to put the tie downs in. This has always been the pilots responsibility to do however the crew member did so trying to help expedite our lift. Because I always release the rotor brake when I remove the tie down bag; in my mind I had released the rotor brake. During my start up procedures I touch each item to verify that I put the item in the correct position. I did this on this startup as well. As I touched the rotor brake I verbalized that it was off even though it was in fact not off; In my mind it was off. I proceeded to start the first engine; allowed it to stabilize at ground idle and checked the engine and rotor gauges. Everything was good so I proceeded to start the second engine and this is when we could smell what we thought was electrical burning smell. I shut down the aircraft and as we opened the door the smell was very intense. We could see smoke coming out the top of the main rotor cowling. Upon inspection I could see the rotor brake smoking and at that time realized the brake was still engaged. [I suggest to] ensure that the medical crew understand the importance of the pilot removing or installing tie downs or other items readying the aircraft for flight.