[Q} how can O.S. predicate a disk going to failure??

[mcclnx mcc <mcclnx@xxxxxxxxxxxx>]

we have CENTOS 4.X on DELL server and one one of virtual disk include 4 disk configure as REID5 (one more disk for hot spare).  I saw /var/log/messages file have:
Aug  4 06:27:02 host1 Server Administrator: Storage Service EventID: 2094  Predictive Failure reported:  Physical Disk 1:5 Controller 0, Connector 1Aug  4 06:27:02 host1 Server Administrator: Storage Service EventID: 2051  Physical disk degraded:  Physical Disk 1:5 Controller 0, Connector 1
I use DELL OPMN to check and found "disk 1:5" still "online", but "predicate failure".
I also use DELL OPMN to check virtual disk and it show "online", not "degrade".
my questions are:
1. is this disk really "degrade" or not?
2. how O.S. can predicate disk going to failure?
3. do I need replace this disk now?
Thanks.

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