HELLO!!
I have a prob in my rb751u-2hnd I was upgrade the os to packages-mipsbe-3.11 and when I reboot it I doesn’t boot 
what I can do to recover the packages-mipsbe-v6.4 ??? plz help me
Hi,
You have to netinstall if ROS not boot. Please read following link:
http://wiki.mikrotik.com/wiki/Manual:Netinstall
thx