Overview of PNP enumeration and hard disk drive letter assignments in Windows Server 2003 and Windows XP

This article discusses Plug and Play (PNP) enumeration in Windows Server 2003 and Windows XP. The article contains information about how Windows Server 2003 and Windows XP assign drive letters to hard disk drives when you run Windows Setup on a computer with multiple hard disks. This article also discusses how PNP enumeration behavior in Windows Server 2003 and Windows XP is different from PNP enumeration behavior in Windows 2000.

When you install Windows Server 2003 or Windows XP on a computer that has two hard disks, where one hard disk is configured as master on the primary integrated device electronics (IDE) channel and where the other hard disk is configured as master on the secondary IDE channel, by default, Setup may assign the logical drive C to the hard disk that is configured as master on the secondary IDE channel. This is a change from the behavior in Windows 2000. When you install Windows 2000 on computer that uses the same hard disk configuration, Windows 2000 Setup tends to assign the logical drive C to the hard disk that is configured as master on the primary IDE channel.

The PNP algorithms that are used by Windows Server 2003 and Windows XP for PNP enumeration are optimized by using an in-place reorder mechanism. The last item that is detected can be listed first. As a result, when you run Windows Server 2003 Setup or Windows XP Setup on a computer with two hard disk controllers, where each hard disk is configured as master, Setup can list the hard disk that is configured as master on the secondary controller first in the list of available hard disk drives for the installation of the operating system to that hard disk.

This behavior can occur on computers that are running Windows Server 2003, Windows XP, and earlier versions of Windows. PNP enumeration and the order of the hard disks that Setup detects is not guaranteed. Note the following considerations that apply to the differences in PNP enumeration behavior in Windows 2000, Windows Server 2003, and Windows XP:

• In Windows 2000, PnP processes its array of requests and moves the hard disk drives that it is going to start first to a separate array.
• Windows Server 2003 and Windows XP perform an in-place reorder operation to optimize PnP. As a result of the reorder operation, Windows Server 2003 and Windows XP may enumerate the hard disk on the secondary IDE channel before the hard disk on the primary IDE channel. The hard disk that Setup detects last is listed first.
  
  The PNP enumeration behavior in Windows Server 2003 and Windows XP applies to the whole operating system and is not limited to computers with two hard disk controllers, as described in the example earlier in this article. For example, this behavior can occur on computers that have multiple logical disks on one Small Computer System Interface (SCSI) hard disk controller or on computers that have multiple network adapters that use the same driver.
  
  For additional information about related topics, click the following article number to view the article in the Microsoft Knowledge Base:
  823206  (http://support.microsoft.com/kb/823206/ ) Unexpected relationship is identified between Device Manager and the local area connections

Important When you upgrade from Windows 2000 to Windows Server 2003, the number that was assigned to a disk in Windows 2000 will not be the same as the number that will be assigned to that same disk in Windows Server 2003. Only the disk number changes; the drive letter does not change.

Note During the text mode Setup of Windows Server 2003, if one of the attached drives is a removable disk drive that contains no media, the empty removable drive is treated as if it has a single logical partition. This drive is assigned a drive letter. If another drive that contains an active primary partition is not found, the removable drive may be assigned drive letter C. To prevent this, make sure that another drive in the system contains an active primary partition. If you use Windows Preinstallation Environment (Windows PE) to start Windows Setup, you can use the Diskpart utility to create partitions before you run Setup.