Breaking a RAID 1 Mirror

December 13, 2005

I recently needed to upgrade a machine that had a mirrored pair of 150Gb Serial ATA (SATA) drives. This was a small form factor (SFF) machine, a Shuttle SB51G XPC, with the drives mirrored via the on-board SATA RAID controller that’s part of the Intel 865G chipset.

What I wanted to do was remove the mirror, leave one of the drives (with its content intact) as the boot drive, and add a 300Gb SATA drive as a (non-mirrored) second drive. In the end, this turned out to be fairly simple to do, but it took me a while to figure it all out.

If you just go into the SATA controller dialog (by pressing Ctrl-I during the boot process), it gives you the option of removing the mirrored volume. Unfortunately, this is accompanied by all sorts of warnings that you are going to lose all of the data on the volume.

Attempting to go into the BIOS settings and set the SATA controller to IDE mode instead of RAID left the operating system (Windows XP Media Center Eddition (MCE)) unbootable. Interestingly, an older XP Pro install on the same machine (via dual boot) would boot fine in this mode, but the MCE install was unable to find a boot device; I’m guessing it’s a difference between driver versions on the two installs.

Breaking the mirror by simply removing one of the drives worked fine (as one would hope it would, that being the point of mirrored drives), and MCE booted up with a warning that the mirror was degraded.

Adding the 300Gb at this point worked, and I was able to see the drive in Disk Manager and format it. Unfortunately, attempting to boot the system afterwords resulted in a “Missing Operating System Error” if the 300 was plugged in. It made no difference which of the two SATA channels each drive was plugged into.

Booting the system again with only the 150 attached, I hot-plugged the 300 (kids, don’t try this at home) and it was visible again, but this obviously wasn’t a workable solution.

The Intel RAID utility told me it could rebuild the volume using the 300, so (for lack of a better idea) I let it do so, mostly because I was curious as to whether the remaining 150 gig would be available to build a partition on (it wasn’t).

At this point, I had three copies of the system — the two original 150s (the one I removed, and the one I left in), plus the 300. I figured that since I could always plug in the other 150, I had nothing to lose by breaking the RAID using the boot-up utility, even if it did lose all of the data on the volume.

I then proceeded to break the RAID, and “surprise” — both drives appeared as seperate drives, with an intact version of the 150 gig system. Despite the warning, no bad things happened.

This left me with a final problem, in that (apparently since I’d left the 300 on the first channel) the 150Gb partition on the 300 was set as drive C, and Disk Manager will not let you change the drive letter of a boot drive. There’s a good reason for this (you can paint yourself into a corner very fast), but if it would have just let me swap drive C to the other drive, it would have been fine.

I know that Partition Magic and a couple of similar programs can fix this, but I didn’t have any of them handy, and at this point I was tired of messing with it and had spent far longer on the problem than I cared to, so I reformatted the 150, and added a second partition on the 300, which left me with the additional space I wanted, albeit on two volumes instead of one.

The only reason I’m bothering to post this is that I found very little information online regarding just what really happens when you break a RAID mirror on a system like this, and maybe I can save somebody else a few hours of grief.

Your mileage may vary, I’d strongly advise backing up first, and I won’t be responsible if you lose all of your data, but that being said, breaking a RAID 1 mirror (at least on a system configured like this one) may not be as fatal as it appears.

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5 Responses to “Breaking a RAID 1 Mirror”

  1. Juan on February 23rd, 2010 1:31 pm

    thank you for this information. I’m tying to figure this out myself while
    going Hhmm…

  2. Chris on March 3rd, 2010 9:53 pm

    Thanks Chuck… I am in pretty much the exact same situation you were, and like you I wasnt able to find much information out there. I got the same warnings when attempting to break my RAID1 configuration, but it didnt equate. Isnt a mirrored configuration supposed to act as a realtime backup? If you cant break one and have two identical copies, then mirroring wouldnt serve much of a function!

    Thanks again.

  3. mike on April 23rd, 2011 2:07 pm

    I’m actually about to break a RAID1 setup right now. I’ve got 2 500GB SATA2’s setup in RAID1 via my Biostar A785G3 motherboard. I created an image via clonezilla, which might be useless ….lol… Also backed up all critical files on another computer. I’m just going to break the RAID and cross my fingers.

  4. Rod on July 27th, 2012 3:11 pm

    Chuck, I want to thank you for taking the time to present this. I had seen other forums suggesting Control I to get to the controls to break the array. Yours just seemed more succinct. However, its not working for me. I have an old (circa 2001) Compaq 8000T running XP pro with a RAID 1 array. One of the drives failed. Frankly, I am having a hard time justifying the expense of replacing the drive and thought I would just remove the failed drive and run the machine on the one. My wife mostly uses it for playing games and checking email. Short of just unplugging the drive and dealing with the constant array failure messages I was hoping to find a way to just break the array. Your solution seemed palatable but hitting Ctrl I at start does nothing. Any clues?

    Thanks for taking the time!

    PS- I could find nothing on Compaq/HP site or the manual that addresses the issue

    Rod Weaver

  5. TJ Lannon on August 1st, 2012 3:56 pm


    I think the use of CTRL I is a function of an Intel software Raid system.

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