Explaining and Troubleshooting Write Cache Disk Drive Letter Assignment

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This blog explains the process involved on drive letter assignments for target devices and how to troubleshoot write cache drive letter changes.

Mount Manager is the component responsible for managing volume names and drive letter assignments on windows. It has a database that is recorded on the registry under the HKLM/SYSTEM/MountedDevices key. When a new disk is installed on the system, the Mount Manager Creates a new unique ID for the disk and saves it into the registry with the “??Volume{GUID} naming scheme, this is unique and persistent for this system. If the disk is plugged on another system, a new unique ID is created. The uniqueid key is stored as a binary registry and has on its data the MBR Disk Signature, which is unique to the disk and will persist among all the systems where the disk will be plugged, for example:

After the volume is formatted and a drive letter is assigned, one new key is created with the letter assignment, it has the DosDevices{Letter} naming scheme, and is stored as binary registry with the same contents as the uniqueid key, as displayed:

For consistency, just one DosDevices{Letter} registry is created for each letter, therefore if a new disk with different signature is added to the system, windows overwrites the registry and assign the letter for the new disk. You come across this behavior when a target device starts as standard on a new target (physical or virtual) that was not cloned from the golden image. To ensure that the new system will be consistent with the drive letter assignment, you must change the unique id for the writecache disk to match the master target device disk.

The following is the procedure to complete the task:

  • On the Golden Image (running as private), assign the letter as required:
    For test purposes, let us use W:

  • Open a command prompt and run the command “diskpart”, on the diskpart command line, run list disk:

  • Select the write cache disk using the command “Select disk {writeCache disk}”

  • Run the command “uniqueid disk” and note the result:

  • Now, Log on the target device that is starting with the different drive letter, as displayed in the following screen shot:

  • Open command prompt and run the same commands used on the Golden Image:

Note: Note that the Disk ID is different.

  • Using the Disk ID from the Golden image, run the command “uniqueid disk ID={ID}” this changes the id to match the same from the golden image.
  • Restart the target device.
  • The Write Cache Drive letter should be the same:

 

 

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