Windows 10: Windows arbitrarily changes a drive letter

Discus and support Windows arbitrarily changes a drive letter in Windows 10 Drivers and Hardware to solve the problem; Windows arbitrarily decides to change the drive letter for my SD card from J back to its default D. Why its default is D, I have no friggin clue, but... Discussion in 'Windows 10 Drivers and Hardware' started by Thanatica, Nov 17, 2019.

  1. Thanatica Win User

    Windows arbitrarily changes a drive letter

    Windows arbitrarily decides to change the drive letter for my SD card from J back to its default D. Why its default is D, I have no friggin clue, but it's wrong for my purposes and that's why I changed it to J.

    Why I need it this way, is of no concern and is irrelevant, atm. I just need to have that drive letter to be J and nothing else.

    So my questions:

    1) Why does Windows change it back? IOW, what circumstances have to happen for a drive letter to be reset to a default?

    2) How can I tell windows to NEVER change a drive letter unless I specifically tell it to?

    It happened to me just now after applying an update that has nothing to do with SD cards, or its host device, or drive letters in general. That is to say, it happened arbitrarily.

    Thanatica, Nov 17, 2019

  2. Cannot change drive letter


    I request you to try the steps below:

    • Press "Windows" + "R" on the keyboard.
    • Type "diskmgmt.msc" and press Enter key.
    • Now right click on the drive and select Change drive letter or path.
    • Select the drive and click on Change.
    • Now select the drive letter and click on OK.

    If you have any further queries, feel free to contact us. We will be happy to help you.

    Thank you.
    Amit(Kumar), Nov 17, 2019
  3. Windows 10 changing USB port drive letters

    Hi Joseph,

    Thank you for posting your query in Microsoft Community.

    I understand the inconvenience you are facing currently.

    I would appreciate if you can provide us the following information to help us understand the issue better.

    • Since when are you facing this issue?
    • Did you make any changes in prior to this issue?
    I would like to inform you that the issue might have occurred due to some changes made to your computer.

    I suggest you to try changing the drive letter of the drive and check if it helps.

    Follow the steps to change the drive letter:

    • Press Windows + X keys and click on Disk Management.
    • Right click on the external drive and click on Change Drive letter and paths.
    • Click on Change the button.
    • Under Assign the following Drive letter, select the desired drive letter.
    • Click Ok to save changes.
    Hope the information helps. Please let us know if the issue persist and we will be happy to assist you further.
    Nachappa C K, Nov 17, 2019
  4. JediNemo Win User

    Windows arbitrarily changes a drive letter

    User Folder Relocation - drive letter changed / can't log in

    Not sure if this is the best thread to post this in.

    Here's the scenario.
    You use the (amazing) folder relocation tutorial in this forum. The process succeeds and you have a wonderful stress free year in Windows 10. Then your motherboard fails and you replace it with a different brand. Suddenly, when you get booted back to Windows, you can't log in! You quickly realize it's because you set your 2nd hard drive to drive letter "D:", but when Windows reloaded the disk drives because of the hardware change, it assigned "D:" to the DVD drive, and now you are stuck!

    Fear not fellow tweakers, I put in the research and have the solution (this may be redundant from another post, tried to search originally but didn't find anything, so my apologies if duplicating).

    Quick detail: First, we are going to boot into the recovery console and use the command prompt. In the recovery console, please be aware that it uses a virtual drive "X:" as the OS drive. Your other drive letters will be changed. Then we will identify which drive letter your OS is installed on. Next we will find which drive letter your user profiles are loaded on. Then we will make registry edits to fix the drive letter changes. Finally a reboot and celebratory dance.

    Note: You will need an understanding of command line operations and registry editing. If you do not, please find that information first, I will assume you have this knowledge moving forward.
    Also, there are no screen shots. I'm sorry, I didn't think I would be posting this while I went through the process, therefore I have no screen shots to share. If anyone has to go through this, it would be great if you could share your experience and provide screenshots if available.


    Step 1: Boot into the Recovery Console and select the Command Prompt option.
    * If you don't know how, basic tutorial: Interrupt the boot cycle 3 times to activate the recovery consol. As the system boots, when you see the windows logo, simply do a hard reset. After 3 times, when the windows logo appears, it will say it is starting "Automatic Recovery". Once that is done, log in with the local admin account and it will (at least for me) automatically open a command prompt.
    * This can also be accomplished a number of other ways including the use of recovery drives, install media, etc.

    Step 2: If you are confident on which drive is likely which, you can skip to step 3. If not, you need to determine which drive is which in the registry. To do this you have to use a variety of steps.
    1. Find the original OS drive. In the command prompt, type the following command:
      bcdedit | find "osdevice"
      The command will return osdecice partition=*: where "*" will be the current drive letter.
    2. Now you need to find the drive that has the user profiles folder assigned, from your folder relocation. There are a few ways to do this, I will list one. Type "regedit" in the command prompt to open the registry editor.
    3. In the registry editor, navigate to HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
    4. In this part of the registry you will see all the drive letters assigned to various volumes. From step 2a, you'll know which volume would be your normal "C:" drive. Write down the hex code associated with that drive letter. Next, you need to determine which drive letters represent volumes on physically mounted drives, and which represent removable media. Not getting too technical, basically you just look for all the volumes that have a hex code length equal to that of the OS drive.
      1. For example, if in step 2a you discovered that your OS drive is assigned the letter "W:", then look at the entry with that drive letter assigned. The number of hex entries represents a volume on a physically connected drive. Find all the other drive letters that have an equal number of hex entries. Those are all volumes on physically connected drives.
    5. Explore each of those drive letters to find the one with the user profile folder.
    6. Do this by typing "dir *:" in the command prompt, where "*" equals the drive letter.
    7. Go back to the registry editor (regedit). Find the "DosDrive" key entry with same drive letter as in step 2e. Write down the hex code for that key entry.

    Step 3: Before we can make the necessary registry changes, you need to connect to a different registry, called a "Hive".
    1. This link has a more expanded explanation of what we are about to do.
    2. In the key folder pane (left pane) of the Registry Editor window, highlight the "HKEY_LOCAL_MACHINE" option.
    3. Next, click on "File" and chose the option for "Load Hive"
    4. You'll have a familiar "open" window, navigate to the os drive found in step 2a.
    5. Navigate to the windows\system32\config folder
    6. Select the "SYSTEM" file option, and click "OPEN"
    7. It will ask for a key name. This is a temporary name and thus doesn't need anything official. Make it easy to remember. I used: "Drive Letter Change". Then click "OK"
    8. Now you will see an option under HKEY_LOCAL_MACHINE for "Drive Letter Change" (or whatever you named it in the previous step.
    9. Open that new folder and navigate down to
      HKEY_LOCAL_MACHINE\Drive Letter Change\MountedDevices
    10. This will look similar to what you found in step 2d, with the notable difference being this is what Windows will look like when not in recovery!

    Step 4: Make the registry changes
    1. Delete every registry entry that either starts with \\??\ OR has a hex code LONGER than the physical drives (Step 2d).
    2. You should be left with \DosDevices\ keys with equal length hex codes.
    3. Find the key with the hex code that matches with the OS drive you wrote down in step 2d. Make sure it is assigned the drive letter "C:"
      1. There should be no change in this
    4. Find the key with a hex code that matches your user folder drive, you wrote this down in step 2f.
    5. Right click that key name and chose the option for "Rename"
    6. Change the drive letter to whichever drive letter is needed for your particular setup.
      1. When I did my folder relocation initially, I had set my user folders to be found on the D drive. So on this step I renamed the key from "\DosDevice\E:" to "\DosDevice\D:"
    7. Confirm there are no duplicate or conflicting key entries: each drive letter will be used only once.
    8. *NOTE* For THIS particular set of keys (MountedDevices), you can safely delete everything thing. Windows will re-initialize these values at boot if it detects drives that are unaccounted for. So, if you delete everything besides the C drive and your user profile drive, that is OK! Windows will reconstruct the missing keys for you. Once you can log in, you can use Device Manager to change drive letters at will. This IS NOT TRUE for all registry items.

    Step 5: Close the registry editor

    Step 6: Type "exit" at the command prompt

    Step 7: Click on the button for "Turn off PC"

    Step 8: Count to 10, take a deep breath, and turn the computer back on

    Step 9: Log into windows

    Step 10: Assuming Step 9 worked, celebrate in your preferred method. If step 9 didn't work, double check you followed my instructions. If not, shut it all down and go to the beach!
    JediNemo, Nov 17, 2019

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