[markw10]

I have a Mac Pro which has 3 internal 1TB Hard Drives. The first drive is the main system drive and the 2nd and 3rd are both used as a 'server drive' which I share and use with all the laptops in my office.
This 2nd and 3rd server drive are set up as a RAID slice.
Everything has gone well except for until around 3-4 weeks ago whenever I would restart the Mac pro it made a strange sound. I don't know how to describe it but at first I thought it was one of the fans on the graphics cards or the fan on the power supply but I ruled that out. It seems that it's coming from one of the hard drives. It only makes the sound at bootup and then it's fine.
I looked at Disc Utility and saw the following:
The 1TB drive that's used as the RAID is listed at the top which it saying just below: RAID Slice for 'Server Drive'

Then it lists my Macintosh HD and below that it lists:
999.86 GB Server Drive
and below that says Server Drive

When I click on 999.86 GB Server Drive it says for RAID Status: Degraded

The type of RAID I'm using is the Mirrored RAID Set

If I click on the RAID tab while selecting the RAID drive I get the following:

999.86 GB 'Server Drive' Mirrored RAID Set: Degraded
999.86 GB Raid Slice (disk0s2)
RAID Slice (and some long number): Missing


I don't know what's going on here. Because of the noise I'm assuming one of the drives is either in the processing of failing or has already failed. From looking at the above information I get the same feeling.

I have a program called Data Rescue 3 which I have run on all 3 drives and it's showing them as healthy.

Does anyone know what the above means? Is there a hardware problem with my drive or is there an issue with a setting on the RAID that can be corrected through OS X?

Thank you for your help.

[P]

It looks like one of the drives in your set has failed (my personal guess is that the servo calibration at startup is failing, and the noise you hear is the drive retrying again and again until it finally gives up). The other drive still works, so you can continue working, but you should replace that broken drive if you want the security that RAID gives you.

EDIT: Backup now, if you haven't already.

[markw10]

Thank you for the advice. I just checked and found the bad drive. It was not vibrating at all and is totally dead. The fortunate thing is it's a Seagate and still under warranty so I'll do the warranty replacement tonight.
When I put the replacement hard drive back into the computer do I have to do anything specific to get it to mirror again and copy the data from the good drive to this drive?

[P]

Depends on whether it's software RAID or hardware RAID. For software RAID, I think you just add the new drive to the old RAID pool in Disk Utility. For hardware RAID, there is a special utility to use - the command seems to be "Make spare".

[markw10]

I set it up through Disk Utility in Mac OS X so does that mean it's software RAID? I hope it is as simple as placing the new hard drive in the system, adding it to the RAID in Disk Utility and it'll mirror the data over. I just wnat to make sure it doesn't erase the data from the current hard drive in the RAID.
I have put in for an RMA exchange of the bad drive and will be replacing it shortly so the information provided will help a lot.
Also, because of this situation I've decided to buy another hard drive so I can also use RAID to mirror the drive in slot 1 so I will then have two sets of mirrored drives.
Currently all 3 drives in my system are Seagate Barracude ST31000340NS 1TB 7200 RPM 32MB Cache SATA 3.0 GB/s. I can get another of these for around $149.
I have found a sale on a Western Digital drive. It is the WD1002FAEX 1TB 7200 RPM 64MB Cache SATA 6.0GB/s. If I'm correct the Mac Pro won't work with 6.0GB/s so it'll run at 3.0GB/s.
Now when I set up a RAID configuation do both hard drives have to be the same size? I know this won't be an issue here since both are 1TB but how closely do they have to match? I know in this case my current hard drive is 32MB Cache and this new one is 64MB cache. Will this cause any issues?
Also, since this newer drive is faster is there a way I can make this a primary drive in my RAID or something like that so that it can speed up my system?
Thank you again for your help with these questions.

[P]

If you set it up in Disk Utility, it's a software RAID 1. The second drive must be at least as big as the current one, and it may not be - there is some range in how large a 1TB drive can be exactly. I don't think the other things you mention will cause any issues.