If you are using a "jtaged" box, the only update you can do is whta you are currently on. i.e. 8955.
Updating to 9199, could do either two things, rrod, requiring an 8955 re-flash, or blow an efuse, meaning no more homebrew.
The media update comes from xbl, and will never be on a disk.
You can find all this info by searching.