Windows – Enable Bitlocker auto-unlock without system drive encryption

I managed to figure out how to fix this. It doesn't use the native auto-unlock function like I was hoping (rather a script that unlocks using a .BEK file, as I mentioned) but it seems to work just as well.

First, you need to create a .BEK file that can unlock the disk. This is the command I used:

manage-bde -protectors -add d: -rk c:\

Substitute d: with whichever drive you want to auto-unlock. The output will look something like this (take note of the "external key file name"):

BitLocker Drive Encryption: Configuration Tool version 10.0.10011
Copyright (C) 2013 Microsoft Corporation. All rights reserved.

Key Protectors Added:

    Saved to directory c:\

    External Key:
      ID: {9DDF4900-5E08-479A-8C4C-6ADE2DA00551}
      External Key File Name:
        9DDF4900-5E08-479A-8C4C-6ADE2DA00551.BEK

Now you need to create the script to unlock the drive. Just create a batch file in the root directory with the following line:

manage-bde -unlock d: -rk C:\[external key filename]

Again, replace d: with the correct drive letter. Replace [external key filename] with the filename you got before.

Finally, you need to tell Windows to run this script at startup, before you log in. (Or at least I needed it to run then, because that's where my desktop, etc. are stored.) Here's how you do that:

1. Press Win+R and enter gpedit.msc. (This is only in Pro, but so is BitLocker, so that's not an issue here.)

2. On the left, navigate to Computer Configuration > Windows Settings > Scripts (Startup/Shutdown)

3. Double-click "Startup" on the right

4. Click "Add..." and select the batch file you created before.

5. Click OK and exit Group Policy Editor.

Now you can reboot the computer to test it. You should see your drive automatically unlocked!