You Cannot Revert a Disk to a Basic Disk If the Disk Appears As "Dynamic Unreadable" in Disk Management

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If the master boot record (MBR) of a disk is damaged or if it is improperly altered, the disk may appear as "Dynamic Unreadable" in the Disk Management snap-in. You can right-click the disk, and then click Revert to Basic Disk to change the disk to a basic disk to facilitate recovery. If you do so, you receive the following message:
Data on the this disk will be lost if you convert the disk to Basic, do you wish to continue ?
If you click Yes, a hour glass appears briefly, and then it goes away. The disk still appears as "Dynamic Unreadable" in Disk Management, and you cannot use the disk to create any new volumes.

If you try to use the Diskpart.exe utility to convert the disk to a basic disk, you receive a "DiskPart successfully converted the selected disk to basic format" message. However, if you perform a rescan operation, the partitions are still intact and the disk still appears as "Dynamic Unreadable" in Disk Management.


The issue may occur if one or more of the partition tables in the MBR contains an invalid partition type. Dynamic disks support only the following partition types:
  • OEM partitions
  • Extended partitions
  • Type 0x42 for dynamic volumes
Other partition table types cause the disk to appear as "Dynamic Unreadable" in Disk Management and prevent Disk Management from converting the disk to a basic disk.

This issue may occur if you delete and re-create a partition on the disk if you are not running Windows 2000 or later. If you use a disk partitioning utility such as DOS Fdisk.exe to repartition the disk, the disk appears as a damaged dynamic disk and you cannot change this disk to a basic disk.

MBR or extended boot record (EBR) corruption may also cause a basic disk to appear as a damaged dynamic disk. This behavior occurs if a hexadecimal 0x42 value is contained in any one of the four partition table system-ID fields in the MBR or in an extended partition's EBR. If Disk Management reads a type 0x42 entry for a partition's system identification (ID), it considers the disk to be a dynamic disk even if the disk never was dynamic.


To resolve this issue, you must first use the Diskmap.exe utility or the Dmdiag.exe utility in verbose mode to confirm that invalid system IDs are causing this problem.

Note: To start the Dmdiag.exe utility in verbose mode, type dmdiag -v at a command prompt.

These utilities are available in the Windows 2000 Resource Kit. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
927229 Windows 2000 Resource Kit Tools for administrative tasks
Note: DMdiag.exe is included in the Windows Server 2003 support tool.
Both utilities output MBR information that contains the system ID or partition type in hexadecimal format. The following sections contain examples of output from these utilities.

Example 1: Output from the Diskmap Command

If you run the diskmap /d1 command from a command prompt, you receive the following output.

Note: Some of the data from the output has been removed.
         Starting               Ending       System    Relative    Total
  Cylinder Head Sector  Cylinder Head Sector   ID      Sector      Sectors
*        0    1    1         509  254   63     0x0B          63    8193087
       510    0    1         891  254   63     0x42     8193150    6136830
       892    0    1        1006  254   63     0x42    14329980    1847475
      1007    0    1        1023  254   63     0x0F    16177456    1590435
In this example, there is a mix of 0x0B (FAT32), 0x42 (dynamic), and 0x0F (extended-LBA) partition types. The partition table entry that causes the issue that is described in this article is the 0x0B (FAT32) partition.

Example 2: Output from Dmdiag Command

If you run the dmdiag -v command from a command prompt, you receive the following output.

Note: Some of the data from the output has been removed.
---------- Partition Table Info Disk 1 ----------

       Starting        Partition     Hidden       Total  Partition    Partition
 Offset (bytes)   Length (bytes)    Sectors     Sectors     Number   Type (HEX)

         32,256    4,194,860,544         63   8,193,087          0         0x42
  4,194,892,800    3,142,056,960  8,193,150   6,136,830          1         0x42
  7,336,949,760      945,907,200 14,329,980   1,847,475          2         0x42
  8,282,857,472      814,302,720 16,177,456   1,590,435          3         0x0F
  8,282,889,728      814,269,952         63   1,590,371          4         0x07
              0                0          0           0          5         0x00
              0                0          0           0          6         0x00
              0                0          0           0          7         0x00
In this example, there are several 0x42 (dynamic) partition types and an 0x0F (extended-LBA) partition type in the MBR. However, the extended partition includes an 0x07 (NTFS) partition table entry in the EBR, which causes the issue that is described in this article.

You can use Dskprobe.exe to make sure that all of the partition table types are either supported or are of type 0x42 (which indicates a dynamic volume). You can also use the clean command with Dskprobe.exe or Diskpart.exe to remove all partitions on the disk.

To use Dskprobe.exe to change the System-ID value to type 0x42:
  1. Log on to the computer as an administrator, and then start Dskprobe.exe.
  2. Click physical drive on the Drives menu, and then double-click the physical drive that you want to repair.
  3. Click to clear the read-only check box for handle zero, click set active, and then click OK.
  4. Click Read on the Sectors menu, click Read to read sector-0 for one sector.

    The partition tables in the MBR appear. The partition table is a 64-byte data structure that is located in the same sector as the MBR at cylinder 0, side 0, sector 1 or physical sector 0 of the physical disk that is using Dskprobe.exe. The partition table conforms to a standard layout that is independent of the operating system. Each partition table entry is 16 bytes long, which makes a maximum of four partitions available. Each entry contains Cylinder, Head, Sector (CHS), and System-ID (XX) information. Each entry starts at a predetermined offset from the beginning of the sector. The following data is an example of the partition table in hexadecimal format:
       000001A0: .....
       000001B0: 00 00 00 00 00 00 00 00 - 00 00 00 00 00 00 80 01 
       000001C0: 01 00 XX 10 7D ED 3D 00 - 00 00 D9 D0 07 00 00 00 
       000001D0: 00 00 XX 00 00 00 00 00 - 00 00 00 00 00 00 00 00 
       000001E0: 00 00 XX 00 00 00 00 00 - 00 00 00 00 00 00 00 00
       000001F0: 00 00 XX 00 00 00 00 00 - 00 00 00 00 00 00 55 AA 
       Partition-1 starts at 0x01BE - SYSTEM-ID Byte is at 0x01C2
       Partition-2 starts at 0x01CE - SYSTEM-ID Byte is at 0x01D2
       Partition-3 starts at 0x01DE - SYSTEM-ID Byte is at 0x01E2
       Partition-4 starts at 0x01EE - SYSTEM-ID Byte is at 0x01F2
  5. Find the partition table entry whose system ID byte is not one of the supported types, and then change it back to "0x42".

    Note: If a logical drive in an extended partition is not one of the supported types, you must browse to the EBR to change this entry.

    The following list describes the supported types:
    • Value: 0x05
      File system: Extended partition (which holds the logical drives).
    • Value: 0x0F
      File system: Same as 0x05 and uses Logical Block Address Int 0x13 extensions.
    • Value: 0x12
      File system: Compaq OEM partition.
    • Value: 0xFE
      File system: IBM OEM Partition.

      Note: If you use this partition, you must apply Windows 2000 Service Pack 3 (SP3).
    • Value: 0xDE
      File system: Dell OEM Partition.
    • Value: 0x42
      File system: LDM Dynamic Disk Partition.
  6. After you manually change the System ID byte to 0x42, click Write on the Sectors menu, and then press Write It to save the changes.
  7. Quit Dskprobe.exe, shut down the computer, and then restart it.
  8. Log on as an administrator, start Disk Management, and then confirm that the disk that appeared as "Dynamic Unreadable" is online.
  9. Revert the disk to a basic disk.
Use the following procedure to use the Diskpart.exe utility to clean the disk.

Note: If you use Windows XP or Windows Server 2003, the Diskpart.exe utility is already installed. If you are using Windows 2000, install the Diskpart.exe utility for Windows 2000. To download the Diskpart.exe utility, visit the following Microsoft Web site:
  1. From a command prompt, type diskpart, and then press ENTER.
  2. Type select disk number (where disk number is the physical disk number that you want to clean), and then press ENTER.
  3. Type clean, and then press ENTER.
  4. Type exit, and then press ENTER.
The disk becomes a basic disk that contains unallocated space. You must allow Disk Management to write a new disk signature before you can use this disk.

More information

For additional information about this issue, click the following article number to view the article in the Microsoft Knowledge Base:
236086 System/Boot Disk Listed as Dynamic Unreadable in Disk Management


Article ID: 320283 - Last Review: June 19, 2014 - Revision: 6.0
Applies to
  • Microsoft Windows Server 2003, Enterprise Edition (32-bit x86)
  • Microsoft Windows Server 2003, Standard Edition (32-bit x86)
  • Microsoft Windows XP Professional
  • Microsoft Windows Server 2003, Datacenter Edition (32-bit x86)
  • Microsoft Windows Small Business Server 2003 Premium Edition
  • Microsoft Windows Small Business Server 2003 Standard Edition
kbenv kbprb KB320283

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