MAYBE THIS WILL HELP
Had an entire post but hit a key and lost it. <SIGH>
So anyway here it is again more or less. I had the same issues some of you had.
First: Basically if you can only see the N7 mounted as the Samsung device, do not worry, it roots fine using that. i
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Well maybe I should say that it worked FOR ME with it listed like that. I simply could not get the naked drives mounted but it was not a big deal in the end. As a matter of fact somewhere along the line it changed to the Android ADB Interface when I just checked I again, weird....
Second I also had the issue of the N7 not seeing the "SD card" and when I did after rebooting the N7 , I did not see the supersu.zip file to install even though the DOS command said it did like the video.
So this will fix both. Just restart the N7 and boot it as normal from the recovery screen.
Let it mount a USB drive, it will be called Nexus 7 in the My Computer list of devices and the drive will be called Internal Storage.
Copy the c:\nexus7\supersu.zip file to this N7 Internal Storage drive. Just stick it in the root along with the folders, Android, DCIM etc...
Then shut down the N7, hold power and Vol down and go from there to install supersu.zip. After I coped the file there manually I could then see the SD card and install it.
Hope this helps.