what is difference between "slow initialize" and "patrol read" on RAID?

[mcclnx mcc <mcclnx@xxxxxxxxxxxx>]

We have DELL server with MD1000 Disk array in it.  O.S. is CENTOS 5.5.  Recently every time MD1000 "patrol read" start I will get "media error" messages on /var/log/message file.  

I use MD1000 "slow initialize" to initialize "bad disk" and NO error.  After "slow initialize" finish, I manually "startup patrol read".  I continue get "media error" on /var/log/message.

Anyone know what difference between "slow initialize" and "patrol read"?

Thanks.
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