Unable to create folder in root of 'Macintosh HD'?

Question

I am running macOS 10.15.4 and I need to create a non-user specific folder at the based of primary volume (Macintosh HD), but Finder does not allow me (option greyed out) and from the terminal I get denied.

From terminal:

mkdir /MyFolder
mkdir: /MyFolder: Read-only file system

Can anyone explain how this can be done? It was definitely possible in the past, before Catalina.

Answer 1

In Catalina the primary volume is read-only. This means it is not immediately possible to create new folders here.

In order to get around that, the system provides what is known as synthetic firm links. This allows you to create what appears to be folders at the root of the file system.

You need to create the file /etc/synthetic.conf, which should be owned by root and group wheel with permissions 0644.

The contents should look like this:

newfolder     Users/foo/bar

"newfolder" is the name of the virtual folder that will be created in the root of the file system. "Users/foo/bar" is the actual location of the folder. You need to ensure that this folder actually exists. It doesn't need to be inside Users, but can be anywhere in your system.

NOTE: It is important to ensure that the space between the two folder names is a TAB character, and not just a number of space.

After creating the file above with the specified contents, you need to run /System/Library/Filesystems/apfs.fs/Contents/Resources/apfs.util -t or reboot the system. After rebooting, you'll see the /newfolder folder.

Answer 2

Here's a copy and paste for terminal to set this up:

# Be sure to change:
# FOLDER_NAME - the root folder name. 
# ACTUAL_PATH_TO_REAL_FOLDER - the path to the "real" folder.
# 
#    Example: 
#    "Drives    Users/bob/Documents/Drives"
#    .......^ this is tab (not spaces)
#    Both of FOLDER_NAME and ACTUAL_PATH_TO_REAL_FOLDER are not starting with /

sudo touch /etc/synthetic.conf
sudo chmod 0666 /etc/synthetic.conf
FOLDER_NAME='...'
ACTUAL_PATH_TO_REAL_FOLDER='...'
echo -e "${FOLDER_NAME}\t${ACTUAL_PATH_TO_REAL_FOLDER}" | sudo tee -a /etc/synthetic.conf
sudo chmod 0644 /etc/synthetic.conf
sudo chown root:wheel /etc/synthetic.conf
/System/Library/Filesystems/apfs.fs/Contents/Resources/apfs.util -t

The apfs.util -t command should allow modifications without requiring a restart, but if it does not work then a restart should make the new entries appear at root.

Answer 3

According to man synthetic.conf:

Because the root mount point is read-only as of macOS 10.15, physical files may not be created at this location. All writeable paths must reside on the data volume, which is mounted at /System/Volumes/Data.

As a work around, the documentation suggests to use /etc/synthetic.conf to create virtual symbolic links or empty directories at the root mount point.

Three examples (via man page)

In each case, append the following to /etc/synthetic.conf

# example 1: create an empty directory named "foo" at / which may be mounted over
foo

# example 2: create a symbolic link named "bar" at / which points to
# "System/Volumes/Data/bar", a writeable location at the root of the data volume
bar   System/Volumes/Data/bar

# example 3: create a symbolic link named "baz" at / which points to "Users/me/baz"
baz   Users/me/baz

Note: the words must be separated by tabs. Unfortunately, StackOverflow renders tabs as spaces.

Automation

echo -e "baz\tUsers/me/baz" | sudo tee -a /etc/synthetic.conf