As part of routine preventive maintenance for your disks, you should
periodically check disks for errors, defragment volumes, and clean up
unnecessary temporary files. Windows
7 provides separate utilities for each of these tasks.
1. Troubleshooting Disk Problems
When you experience problems with a disk, you can use Disk
Management to help you troubleshoot. In most cases, partitions and
simple volumes are easier to troubleshoot and recover than mirrored,
spanned, and striped volumes. With partitions and simple volumes, only
one disk is involved. If a disk with a partition or simple volume has
problems, you might see the Failed, Online (Errors), or Unreadable
status. This occurs because after a certain number of errors, Windows
flags the disk, and although this is likely a sign of a serious problem,
you can attempt to use the disk by right-clicking the volume and
selecting Reactivate Disk. If this doesn’t work, click Rescan Disks on the Action menu. If a disk is listed as
Failed or Unreadable and won’t return to a Healthy status, you should
replace the volume. If a disk is listed as Online (Errors) and won’t
return to an Online status without errors, you should check the disk for
errors. Keep in mind that
a disk with recurring errors is likely on the way out and will likely
stop working eventually.
Sometimes you might need to reboot your computer to get a disk
back online. The Online (Errors) status can also be an indicator of a
failing disk, so if you see this status several times on the same disk,
check for problems with the drive, its controller, and its
In a desktop or server computer, a bad power supply could also
be the source of the problem, so make sure that the components in your
computer (drives, video cards, CPU, etc.) do not draw more power than
your power supply can handle. Excessive heat and inadequate space for
ventilation can cause a power supply to operate below its rated
wattage. You may need to upgrade to a more capable power
With disks that have mirrored, striped, and spanned volumes, the
drive status might show as Failed, Online (Errors), or Unreadable. In
many cases, you can resolve these problems using the same techniques as
with simple volumes. Right-click the volume and select Reactivate Disk.
If this doesn’t work, click Rescan Disks on the Action menu. You might
also see the Missing or Offline status if drives have been disconnected
or powered off. In this case, you can try to reactivate or rescan, but
this probably won’t work. To resolve the problem, you may need to check
the disk to ensure that it is connected and that its power supply is
connected. If you can’t fix the problem, you’ll need to replace the
disk. Shut down your computer before you try to examine your computer’s
2. Breaking or Removing Mirroring
You may want to break a mirrored set or remove mirroring.
Breaking a mirrored set and
removing a mirror are two very different operations. When you break a
set, you stop mirroring and break the mirrored set into
two independent volumes—both of which contain all the original data.
When you remove a mirror, you stop mirroring and remove all data from
one on the volumes so that you can use it for other purposes.
You also may need to break a mirrored set to repair a mirrored
set. For example, if one of the mirrored drives in a set fails, disk
operations can continue. However, at some point you’ll need to fix the
mirror, and to do this you must break the mirror and then reestablish
In Disk Management, you can break a mirrored set by following
Although breaking a mirror doesn’t delete the data in the set,
you should always back up your data before you break the mirror.
This ensures that if you have problems, you can recover your
Right-click one of the volumes in the mirrored set, and then
click Break Mirrored Volume.
Confirm that you want to break the mirror by clicking Yes. If
the volume is in use, you’ll see another warning dialog box. Click
Yes to confirm that it’s OK to continue.
Windows breaks the mirror, creating two independent
In Disk Management, you can remove one of the volumes from a
mirrored set. When you do this, all data on the mirror you remove is
deleted, and the space it used is marked as Unallocated. To remove a
mirror, follow these steps:
In Disk Management, right-click one of the volumes in the
mirrored set, and then click Remove Mirror. This displays the Remove
Mirror dialog box.
In the Remove Mirror dialog box, select the disk from which to
remove the mirror.
Confirm the action when prompted. All data on the removed
mirror is deleted.
3. Resynchronizing and Repairing a Mirrored Set
As part of the mirroring process, Windows is constantly
synchronizing the data on mirrored drives. When problems arise, however,
data on mirrored drives can become out of sync. For example, if one of
the drives goes offline, data is written only to the drive that’s
You need to get both drives in the mirrored set online. Because a
disk in the set has failed, the mirrored set’s status should read Failed
Redundancy. You can use various techniques to resynchronize and repair
mirrored sets. The corrective action you take depends on the failed
When a drive has a status of Missing or Offline, ensure that
the drive has power and is connected properly. Then start Disk
Management, right-click the failed volume, and then click Reactivate Volume. The drive status should change to
Regenerating and then to
Healthy. If the volume doesn’t return to the Healthy status,
right-click the volume, and then click Resynchronize Mirror.
When a drive has a status of Online (Errors), right-click the
failed volume, and then click Reactivate Volume. The drive status
should change to Regenerating and then to Healthy. If the volume
doesn’t return to the Healthy status, right-click the volume, and
then click Resynchronize Mirror.
When one of the drives shows a status of Unreadable, you might
need to rescan the drives on the system by selecting Rescan Disks from Disk Management’s Action menu. If
the drive status doesn’t change, you might need to reboot the
If you’ve tried the previously listed techniques to repair the
mirrored set and one of the drives still won’t come back online,
right-click the failed volume, and then click Remove Mirror. Next,
right-click the remaining volume in the original mirror, and then click
Add Mirror. You now need to mirror the volume on an unallocated area of
a different drive. If you don’t have unallocated space on another drive,
you need to create space by deleting other volumes or replacing the
failed drive. Note that you must rebuild the set using disks with the
same partition style—either MBR or GPT.
4. Repairing a Mirrored System or Boot Volume
When you’re mirroring the system or boot volume, or both,
and the primary mirror drive has failed, the drive failure might prevent
your system from booting. Don’t worry: Windows 7 should have added an
entry to the system’s boot manager that allows you to boot to the
secondary mirror. Resolving a primary mirror failure is much easier with
this entry in the boot manager file than without it, because you can
easily boot to the secondary mirror and then repair the problem. If you
mirror the boot volume and a secondary mirror entry is not created for
you, you can modify the boot entries in the boot manager to create one
using the BCD Editor (Bcdedit.exe).
If the primary mirror fails and your computer has a secondary boot
entry, restart your computer and select the Boot Mirror–Secondary Plex
option for the operating system you want to start. Your computer should
start up normally. After you successfully boot the computer to the
secondary drive, you can rebuild the mirror with these steps:
Break the mirror set, and then re-create the mirror on the
drive you replaced, which is usually Drive 0. Right-click the
remaining volume that was part of the original mirror, and then
click Add Mirror. This displays the Add Mirror dialog
In the Disks listbox, select a location for the mirror, and
then click Add Mirror. Windows 7 begins the mirror creation process.
In Disk Management, you’ll see a status of Resynching on both
volumes. The disk on which the mirrored volume is being created has
a warning icon.
If you want the primary mirror to be on the drive you added or
replaced, use Disk Management to break the mirror again. Make sure
that the primary drive in the original mirror set has the drive
letter that was previously assigned to the complete mirror. If it
doesn’t, assign the appropriate drive letter.
Right-click the original system volume, and then click Add
Mirror to recreate the mirror.
Check the boot configuration and ensure that the original
system volume is used during startup. You may need to modify the
boot configuration to ensure this.