How to automount an external vhdx file in WSL2

vhdx.txt

1. Ubuntu WSL2 must be already installed in C: system drive and user should be able to call windows binaries like wsl.exe from bash.

2. We will install Alpine WSL2 distro in an external partition/disk:

    Download an Alpine ZIP file from here:
    https://github.com/yuk7/AlpineWSL/releases/

    Extract the files in an external partition/disk. (for example D:\Alpine)
    Make sure WSL2 is enabled by default (wsl.exe --set-default-version 2)
    Inside the Alpine folder run `winpty Alpine.exe` to install the Distro. An ext4.vhdx file will be created in that same folder.
    Run `winpty Alpine.exe` again.
    Now from the Alpine terminal we will create same user as Ubuntu:
        `addgroup -g 1000 nevmerzhitsky`
        `adduser -u 1000 -G nevmerzhitsky nevmerzhitsky`
    This will create the /home/nevmerzhitsky folder.
    
3. On Ubuntu edit the ~/.profile file and add:

# mount external /home folder from Alpine distro
if [ ! -d /mnt/wsl/Alpine ]; then
    mkdir /mnt/wsl/Alpine
    wsl.exe -d Alpine mount --bind /home/onoma /mnt/wsl/Alpine/
fi

4. We will close the VM with:
    wsl.exe --shutdown

5. Next time Ubuntu distro is launched it will automount the /home/onoma folder from Alpine distro to /mnt/wsl/Alpine in Ubuntu with all
the benefits and speed of ext4 mounted folders.

How it works
---------------
WSL2 runs a single lightweight VM that supports multiple distros running on the same Linux kernel. 
Every time a distro is launched, its vhdx file is attached automatically to the VM as a /dev/sdc device.
Thanks to the nature of WSL interop we can launch another distro and inmediately close it from inside bash.

The folder /mnt/wsl is an undocumented WSL2 feature which is a tmpfs special folder used by applications as Docker for Desktop.
Everything mounted in that folder will also appear in every running distro under /mnt/wsl/. Just make sure you have file permissions to 
access that directory. In my case I'm using user "onoma" on both Ubuntu and Alpine.

In Windows 10 19042.906 it works unstable and mounted /mnt/wsl/Alpine/ will just disappear from time to time. After fully restart of WSL subsystem it appears back.