hold that thought
im not sure you need root to mount sdcard.
its a little different with samsungs to mount sd via ADB. it doesnt respond to to adb shell mount /sdcard/
or mount /sdcard like htc devices do.
i think you have to do some mad stuff with ums.
more testing is in order here, last thing we want to do is void your warranty with no return.
the fact its not charging and only lighting up the charge icon concerns me as there may not be enough power for download mode to work properly.
im gonna do some tests
Samsung Galaxy S3 (playing with roms still)