How Windows 2000 Assigns, Reserves, and Stores Drive Letters
The information applies to:
In Windows 2000, all drive letters assigned to volumes are assigned by the Mount Manager (MountMgr) program. After a drive letter is assigned to a volume by MountMgr, the drive letter is reserved for the volume in the MountMgr database located in the system's registry. Windows 2000 Setup also uses MountMgr to handle BASIC, Dynamic, and legacy FT-Set drive letter assignments during a clean installation and are displayed and assigned in a specific order, sometimes based on past drive letter assignments.
When you perform a Windows 2000 upgrade from Windows NT, the drive letter assignments are
exported to the $Win_nt$.~bt\Migrate.inf file for text-mode setup to reference and "import"
into the MountMgr database as part of the upgrade process. This process occurs even if you are installing
into a new folder, but only if that new installation was started while you are in Windows NT or Windows
2000. (The installation can be performed from the autorun pop-up dialog box when the installation CD
is inserted or by opening the i386 folder and running winnt32 from the command prompt.)
Windows 2000 MountMgr assigns drive letters to BASIC disk partitions according to the following rules.
NOTE: This table is used for volumes which do not already have a drive letter assignment reserved in the MountMgr database. This table would primarily be used during a fresh Windows 2000 installation. Otherwise, after setup is complete, if you add a group of drives, the rules below will be applied to those volumes which do not already have a drive letter assignment.
The following are the basic disk drive letter assignment rules for Windows 2000:
NOTE: In Windows 2000, all legacy FT-Sets on basic disks record their last used drive
letter in a private region on the physical disk. During a new installation, these drive letters
are extracted and favored when re-assigning drive letters. This means legacy FT-sets are assigned
drive letters prior to assigning any other drive letters to other basic disk partition as stated
All assigned drive letters are "persistent" after they are assigned. This means they should remain assigned to that volume until it is either deleted or changed manually using Disk Management. This should hold true for all online volumes as you add and remove disks to the system.
The exception to this rule is that if a volume (disk) is offline, and a different new volume comes online, it may get the offline volume's drive letter. To ensure you keep your drive letter assignments intact, you must keep existing volumes online when introducing new volumes.
NOTE: Disk Management Console prohibits changing the Windows 2000 System/Boot partition drive letter. This is by design.
Dynamic Disk Considerations
Logical Disk Manager (LDM) keeps track of the last drive letter assigned to a dynamic volume by recording it in its configuration database. The LDM configuration database is located in a 1 MB private region on the end of each dynamic disk. This drive letter is then passed to MountMgr when MountMgr asks LDM for a "suggested" drive letter before assigning one to the volume.
DYNAMIC Disk - Drive Letter Assignment RulesMountMgr assigns a drive letter to dynamic volumes according to the following rule:
Importing Foreign Dynamic Disks
After you import foreign dynamic disks, more than likely the MountMgr database on the local system does not have records for those volumes when they are brought online. However, those same volumes may have had drive letters assigned to them when they were used on the other system and LDM recorded those drive letters in its configuration database at the end of the physical disk. When the volume is mounted for the first time, LDM "suggests" the old drive letter used by that volume to MountMgr. If the suggested drive letter is not used, the volume gets the drive letter suggested, otherwise it gets the next available drive letter.
Dynamic Disks During Setup
Windows 2000 currently does not fully support pure dynamic disk volumes during Setup. Setup only understands
dynamic disk partitions that are hard-linked. Hard-linked dynamic partitions are created when you upgrade a BASIC
disk to dynamic, and the basic disk had pre-existing primary and or logical drives configured. Each of the these
pre-existing partitions retain a legacy-style partition table entry (type-42 for primary and type-05 for extended)
even after you upgrade the disk to dynamic. These special hard-linked partition table entries allow Setup to recognize
them as valid partitions in which the operating system can be installed.
Pure dynamic disks (those not containing any hard-linked partitions) only have a single partition table entry (type 42) to define the entire disk. Volumes contained on a pure dynamic disk are called soft-linked, and do not display drive letters for them until Setup is complete. Windows 2000 Setup displays a single drive letter for the whole drive (usually the drive letter assigned to the first soft-linked partition) even if it had multiple volumes configured. You cannot perform a new installation to this type of dynamic disk without destroying underlying volumes.
Considering the above, for a new installation of Windows 2000, the MountMgr database in the system registry is newly created. Drive letters for dynamic volumes are enumerated first and behave similar to importing foreign dynamic disks. This means suggested drive letters from LDM are assigned to dynamic volumes prior to scanning and assigning drive letters to basic disks but after legacy FT-Sets are assigned. For remaining dynamic partitions that cannot be assigned their suggested drive letter, they are assigned a letter based on the same rules as for basic disks. Dynamic hard-linked partitions are equivalent to a basic primary and dynamic soft-linked are equivalent to basic logical drives.
In the case of an upgrade, Winnt32 Setup copies the pre-upgrade MountMgr database to the $Win_nt$.~bt\Migrate.inf file for text-mode setup to use, so the drive letters should be preserved for dynamic volumes.
Please be aware of the following issues:
Q221799 Drive Letters Assigned to Unsupported Partition Types
Q227364 Dynamic volumes are not displayed accurately during Textmode Setup or Recovery Console
Q227605 Windows 95 Drive letters may not migrate to Windows 2000 during upgrade
Q225025 Setup Changes Drive Letters After a Partition Is Deleted
NOTE: If you are dual booting between Windows 2000 and MS-DOS or other versions of Windows, the
drive letters assigned to partitions may not match.
For more information on how other operating system assign drive letters please reference the following Article:
Q51978 Order in Which MS-DOS and Windows Assign Drive Letters
Based on all of the above information, all drive letters are displayed and assigned in the following order during a new installation of Windows 2000 or booting to the Recovery Console.
Courtesy Microsoft Corporation
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