Hard Drive Failure?

I have 3 Western Digital 320GB hard drives installed in my computer.  The first 2 are setup in a Raid 1 configuration and are working ok.  I had the 3rd drive installed back in July to use for periodic backup and/or misc. temporary information storage.  I can't remember what I have stored on the drive, if anything, but now it is no longer being recognized.

When I startup my computer, as it goes through the startup checks, all 3 drives are listed as follows:

Port 0     OFFLINE MEMBER (shown in red lettering)

Port 1     MEMBER DISK (0) Raid (shown in green lettering)

Port 2     MEMBER DISK (0) Raid (shown in green lettering)

The above may not be exact, but I hope you can interpret.  Once Windows is up and running, the 3rd drive (Port 0) or D: Drive is not displayed at all therefore cannot be accessed.  Has the 3rd hard drive failed?  I've never had a hard drive failure before so I don't know how to recognize a failure.  I thought that it may be a power problem, maybe a Sata cable came loose or maybe a BIOS malfunction.  I'm going to check the power cable tomorrow and also to run my computer with the side panel off to see if I can feel any vibration from the drive to determine if it is spinning or not.

I would appreciate any input from any computer techs out there.  Luckily, it wasn't one of my RAID drives, however this is why I setup a RAID system in the first place.

My motherboard is the ASUS P5K Deluxe WiFi, I believe with Award BIOS, but don't quote me on the BIOS.

Thanks,

PattayaDavid