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

Wed Jun 8 19:00:48 UTC 2011
mcclnx mcc <mcclnx at yahoo.com.tw>

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.