Other solution :Remove the bad board
Overclock the S2
Run the Overclocked S2 with only 9 boards
I will have the same hashing speed as 10 boards

What frequency are you running? I facepalmed myself just now for not thinking of this!
option 'freq_value' '0801' #212.5M
option 'chip_freq' '212.5'
option 'timeout' '40'