Software mirrored volumes in Windows Server 2008 may not automatically resynchronize when a member disk is removed from a system and subsequently re-added

Source: Microsoft Support

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Consider the following scenario.� You have a system running Windows Server 2008, and this system has two Dynamic Disks containing a volume that is being software mirrored by Windows.� You remove one of the disks in the mirror from the system, and later re-add it to the system.

When the disk is re-added to the system, it may not automatically be resynchronized with the other disk in the mirror, and the�re-added�disk may appear as "Dynamic Foreign" in Disk Management.

This behavior is by design.

Dynamic disk groups in Windows Server 2008 use a quorum model to prevent disk groups from gaining online state on more than one machine, and to ensure�the most recent copy of the data in the mirror is being used for mirror resynchronization should resynchronization need to occur.

Disk quorum is maintained when there are n/2+1 dynamic disks present in the system, where n is the�number of disks in the disk group.� For a mirror configuration with two disks, this means two dynamic disks must be present in the system at all times for quorum to be maintained.� Therefore, if there are two dynamic disks in a system and one of the disks is removed from the system, quorum is lost, and Windows�cannot�determine which of the two disks contains the most recent copy of the mirrored data.��Because of this, Windows changes the internal identifier of the dynamic disk group containing the mirror volume, which causes the second disk to appear as foreign when it is re-added to the system.

To prevent the behavior described in the "Symptoms" section, ensure n/2+1 dynamic disks are present in the system at all times, where n is the�number of disks in the disk group.

If a member disk needs to be removed from the system, ensure enough dynamic disks are still present in the system to maintain quorum once the disk is removed.� For example, for a mirror volume with two member disks, there should be three dynamic disks present in the system.� This will ensure two dynamic disks (n/2+1) remain in the system once a member disk is removed.� This will maintain quorum, and thus the removed disk will automatically be resynchronized once it is re-added to the system.

If you are already seeing the behavior described in the "Symptoms" section, please follow these steps to bring the mirror back to a healthy state:

1. Open Disk Management.
2. Right click on the disk showing as Dynamic Foreign, and select "Import Foreign Disks..."
3. In the Import Foreign Disks dialog box, select the foreign disk group, then click OK.��Click OK again in the resulting dialog box.
4. Both mirror volumes will be converted to Simple.� On the disk with the older copy of the mirror data, right click the simple volume that was formerly in the mirror, and choose "Delete Volume..."
5. Right click the volume that contains the more recent copy of the mirror data, and choose "Add Mirror..." to add�the disk from step 4 to a mirror of the volume.