the server is remote, cannot see the disk's LED light.
The Raid Controller is motherboard integrated. 6 disks build a Raid 5. Windows 2003 Server system.
so if one disk failed, how the admin get aware ?
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Most servers have hardware monitoring software available. Check with your vendor to see if these exist for your hardware, they should be able to give you drive status inside Windows. Dell's Open Manage is an example of this.
Since you provided basically no detail, this is as good of an answer as I am able to provide.