I had the same problem at two from 8 machines with a fresh 7.0 install. All machines are identical machines/setups.
No smartdisk failures or other errors, only timeout and then lost mirror
I scratched my head over 3 weeks with raid rebuilds since i could see the device in /dev, null it with dd and rebuild.
One day a power supply from one of the affected machine smokes away, after a replace the error was gone.
The solution for the two machines was a power supply replacement.
Maybe this can be also the case for you?