Installing MongoDB on Mac Catalina using Homebrew

I recently bought a new iMac and moved all of my files over using Time Machine. The migration went really well overall and within a few hours I had my development machine up and running. After starting an application I’m building I quickly realized that I couldn’t get MongoDB to start. Running the following command resulted in an error about the data/db directory being read-only:

mongod --auth

I tried every chmod and chown command known to man and woman kind, tried manually changing security in Finder, compared security to my other iMac (they were the same), and tried a bunch of other things as well. But, try as I might I still saw the read-only folder error when trying to start the server….very frustrating. I found a lot of posts with the same issue but they all solved it by changing security on the folder. That wasn’t the problem on my machine.

After doing more research I found out that Catalina added a new volume to the hard drive and creates a special folder where the MongoDB files need to go. The new folder is:


The MongoDB files can then go at:


I ran the following commands to install the latest version of MongoDB using Homebrew (see for more details):

brew tap mongodb/brew
brew install mongodb-community

I then went into the MongoDB config file at /usr/local/etc/mongod.conf. Note that it’s possible yours may be located in a different location based on how you installed MongoDB. I changed the dbPath value to the following and copied my existing DB files into the folder:

dbPath: /System/Volumes/Data/data/db

Finally, I made sure my account had the proper access to the folder by running chown (something I had tried many times earlier but on a folder outside of /System/Volumes/Data):

sudo chown -R $USER /System/Volumes/Data/data/db

After that I was able to start MongoDB and everything was back to normal. Hopefully this saves someone a few hours – I wasted way too much time on the issue. 🙂

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