I/O error while rebuilding an md raid-5 array

[Dawning Sky]

Hi,

I have some trouble with my md raid-5 array.  It has four 500GB drives
(sdb1-sde1).  sde started giving some SMART error (Pending Bad Sectors), so
just to be safe, I decided to replace it.  I declared it to be faulty and
removed it from the array and add a new drive and the rebuilding was
automatic.  But before the rebuilding can finish, I got an I/O error from
sdb1 and it was declared faulty by md.

Now I have two faulty drives and things don't look good.  However, I was
able to added the second drive back to the array and md seemed not mind and
still reports "active sync".  At this point I shut down computer and decided
to clone sdb with clonezilla.  Not sure if it will complete without I/O
errors.  It appears clonezilla is using dd and the speed is extremely slow
(~5MB/sec) and it says it's gonna take 1 day to clone the 500GB.

The only reason I'm not in a total panic mode is that I did a back up before
doing all this.  Now I'm keeping my finger crossed that my backup drive
won't die.  In retrospect, I should have just shut down the computer and
cloned sde instead of letting md to rebuild the array.

Any suggestion on the best to proceed is highly appreciated, especially on
the scenario that I won't be able to clone sdb.  Is there any way to avoid
building a new array?

Regards,

DS

PS, if in the end I have to build a new array, I'll probably go with a raid
6 instead.
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