Do Chinese customs agents check the contents of External HDD and Laptop

Will Chinese airport security/customs ever check the contents of an external hard drive? If so, under what circumstances would they do this and why?

And yes, I have lot of movies, music CD Rips and work related software in my external HDD. (No porn, no illegal distributions copies or any propaganda etc. lol)

I own some musical instruments, but no plans to bring them. How about the customs clearance procedure for Used laptop computers? Does it come under the “channel of non-declaration”

I'll be arriving China with a Z Visa, which box should I tick against certain regulation.

Lot of queries, sorry! it's just that this is my first time travelling to another country.