Software Raid 5 MD0 just stopped working

I'm helping a friend with an old FC6 system I set up for him ages ago.

It has a Logical Lolume made from MD0 and MD1 that in turn are two three 
disk raid 5 sets.

One day MD0 decided not to play any more. When I looked at the system 
MD0 was no longer mentioned in /proc/mdstat. And the VG was showing that 
it was made of an unknown device and MD1.

I reassembled MD0 and the Raid appeared to be happy and re-established 
the uuid of MD0 and the LV was found again but the ext3 filesystem on 
the LV was a shambles. Its all backed up so it can all be put back.

The machine runs smartctl -a on its disks daily and I have the records 
of that going back for over a year. MD0 is made of two Western Digital 
500G's  and a Seagate 500G. All the smartctl data looks fine. Except 
that the Seagate is showing:-

ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
   1 Raw_Read_Error_Rate     0x000f   114   099   006    Pre-fail  Always       -       70478277
   3 Spin_Up_Time            0x0003   094   093   000    Pre-fail  Always       -       0
   4 Start_Stop_Count        0x0032   100   100   020    Old_age   Always       -       26
   5 Reallocated_Sector_Ct   0x0033   100   100   036    Pre-fail  Always       -       0
   7 Seek_Error_Rate         0x000f   081   060   030    Pre-fail  Always       -       126904381
   9 Power_On_Hours          0x0032   061   061   000    Old_age   Always       -       34946
  10 Spin_Retry_Count        0x0013   100   100   097    Pre-fail  Always       -       0
  12 Power_Cycle_Count       0x0032   100   100   020    Old_age   Always       -       26
187 Unknown_Attribute       0x0032   100   100   000    Old_age   Always       -       0
189 Unknown_Attribute       0x003a   100   100   000    Old_age   Always       -       0
190 Temperature_Celsius     0x0022   069   060   045    Old_age   Always       -       554958879