I found that the first track(whether it is an audio track or not) always start at sector 150. I wonder why the first 150 sectors are preserved. Are there existing some "hidden" tracks? Any help will be appreciated, thanks!

I think it's just part of the spec, a mandated lead-in much like the silent 90 second mandatory lead-out after the last track. I don't know the reasoning behind it...

According to people more in the know than I, the first 150 frames is actually called the pause area and the real lead-in comes just before before it.

http://www.digital-inn.de/129475-post19.html