Ok ho provato a rifarlo e vi posso dare qualche altra info, con il log:
Mounting system RW and pushing SU, then remounting system RO again
mount: Operation not permitted
failed to copy 'su' to '/system/xbin/su': Read-only file system
Unable to open /system/xbin/su: No such file or directory
Unable to open /system/xbin/su: No such file or directory
mount: Operation not permitted
Installing superuser
3753 KB/s (2025538 bytes in 0.527s)
pkg: /data/local/tmp/superuser.apk
Success
Cioè praticamente non me lo fa copiare perché dice read-only file system...mah