I want to know how to access hard drive after ExitBootServices() without assembler code, but I can't get sufficient information from uefi spec (one reason is that I can't read English very much.) Can anyone tell me the way to control medium? Is it possible to take over functions from firmware after BootServices() called?
There's no way to do it without writing your own storage, disk and partition drivers because once you call ExitBS() you become a sole owner of the initialized HW and can act as the first stage of OS kernel.
Writing storage drivers for your own OS kernel is out of scope for UEFI.