What is OS X Base System?

This is a question that has been asked by many and if you go looking for the answer it can be a little unclear and confusing. You may have seen this on your Mac either in the disk utility or if you have been poking around the hard drive in a terminal session. What is it?

For the most part, it’s not something you need to worry about unless you are trying to restore, recover or repair your Mac back. Some have wondered if they can remove it to reclaim the disk space but in my opinion, it’s not a good idea and it’s not worth it.

My name is Eric and I have been an avid computer user since the late 1980s. While most of my past has been spent working on Windows PCs, I have recently been using, learning, and researching Mac systems, and as I learn new things I like to provide the information to others.

While, the OS X Base system on a Mac, is not something that everyday users work with or even need to worry about, many are curious as to what it is all about. If you would like to find out more, keep reading and we will discuss this a little further.

My Experience with OS X Base System

I have, of course, had some experience with Mac systems over the years, but it has only been the last couple of years that I have started working with and exploring the Mac world. As an engineer and being fairly new to the systems, I am always curious when I encounter new things.

I first saw a Base System directory when I was poking around the files and directories from a Terminal session on my Mac. I later saw this as well when looking at the disks and volumes in recovery mode and wondered what it was and what is for.

As it turns out, I was actually using it and didn’t even know it.

Investigating OS X Base System

Being curious as to what the Base System is, the first thing I did was a Google search on it. In doing that I found that there is not a lot of information out there as to what this is and what it does. 

The only thing I could find was some very general statements indicating that it has something to do with Mac’s Recovery Mode. That makes sense, but I then wanted to know if it is related to the actual recovery mode itself or is an image that is used to reinstall or repair your mac.

I was first thinking that maybe this is the base operating system that your computer is shipped with, so if you do a factory reset, it will use this to create the new fresh macOS that your Mac originally had. That sounded good, but I was still unsure so I did some more digging.

I posted this question online in a Mac support forum but received only one response. While that response did not give me a whole lot of information it did point me in the right direction and spark something that made sense.

The size of the OS X Base System on the disk is much too small to be an entire OS such as Catalina, Big Sur, Monterey, or whatever your system was shipped with. I’m sure the designers would not want to take such a huge chunk of disk space just to keep the original OS intact.

Besides, recovery mode does allow you to connect to the internet where you can easily download an OS image and install it. It also lets you access a flash drive or a USB hard drive if you have a backup stored on one of those. There’s no need to store it on the actual computer.

OS X Base System Findings

My next thought was that if OS X Base System is not the original OS that the Mac was shipped with, maybe it is just a small simple operating system that is used to run the recovery mode. This would be a small image, so the size of the OS X Base System would make sense.

When thinking about it, recovery mode must have some type of small, slimmed-down OS that can boot the system up, perform a few maintenance functions, and has utilities needed for recovery of your Mac in the case it needs restoring. 

After doing more research and getting input from others, it appears that this image is only around 2Gb and is located on the boot ROM. This ensures that it is always there since we can’t delete things from ROM and is available no matter what happens to our hard drive.

So, from what I have found, I do believe that OS X Base System is the small, slimmed-down operating system that is made available when you go into Recovery Mode. Recovery mode is what is running in the image above.

What is Recovery Mode?

If you have never used it, you may be wondering what Recovery Mode is. It is a mode that you can boot your Mac into at startup which allows you to have internet access, access to disks, and other utilities you can use to repair or restore your Mac.

It’s not only used for recovering from a disk failure, but one may also use this to restore their Mac to its original factory settings with a new clean version of your operating system. This might be done if you are experiencing issues with your Mac that you have been unable to fix.

It might also be done if you are going to sell your Mac or give it to someone. You might want to wipe the system clean so that they don’t have any of your data or information and also so they have a fresh clean system to start with.

You can find more information on Apple’s recovery mode and how to get your system into it from Apple Support.

Can I Remove OS X Base System?

Many who are looking to gain more disk space will often ask if they can remove or delete OS X Base System in order to gain more free disk space. This is a great question and my advice would be that you should not try this.

First of all, the space used by the OS X Base system is only around 2Gb, so you’re not gaining that much and you would be losing a critical function of your Mac that is needed if you ever need to repair or re-image your computer. Without it, things would be very difficult.

Also, I do not think it is possible to delete it because it is stored in ROM (short for Read-Only Memory) which is kind of like permanent storage that you can’t just write to or delete from like you do with your hard drive. It can only be modified with specific software.

The files stored in ROM are then mounted and shown as a volume on one of your disks. So if you do delete it, you would just be deleting the mount point and not the actual files. This would end up giving you little or no new disk space.

So I wouldn’t recommend doing this for any reason. You won’t gain any significant usable disk space and it will only make things more difficult if you need to restore or repair your Mac in the future.

Final Words

As we have seen above, the OS X Base System on your Mac is the operating system that is used to start up and repair, recover or reinstall your main operating system. There is not a lot of information available about this area of your computer but it is a critical part of it.

If you have more information on what OS X Base System is and how it works, I would love to hear from you! I am always looking to learn more about things like this.

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  • Edwin

    Hi Eric, I just read your explanation about the OS base system. Very interesting. I am also a curious person who always wants to explore everything in depth. I was wondering if it’s actually in a ROM or if it’s installed on a hidden partition on the internal SSD, so on the same physical hardware as the operating system. If I choose the option “Erase all data and settings” in Ventura or Monterey, then I can just start all over again, but then I will not enter recovery mode. So I can’t also start my disk management before I start installing. (and therefore also not install another OS version from USB.

    • Eric

      Hi Edwin,
      I am glad that you had a chance to read the article and found it interesting. It is great to be curious and explore issues like this and I am grateful to get your feedback and questions as it helps me to learn about these topics in more depth also. I wrote this article quite a while ago and at the time my resources were explaining the idea of the OS base system existing on a ROM and that data from the ROM being mounted to your local drive. So deleting it from your local drive would only be deleting the mount point but the data would still exist on the ROM. I am trying to go back and find the original information that I used for this research but I am not finding it. It could be that this is the way it was done on older systems and is no longer the case. I am going to go back to see if I can find my original sources, but in any case, I believe that it is never a good idea to delete or remove the OS base system. It doesn’t take up enough space to really make a difference. At least not enough where it is worth it to cause issues with your system as you describe above. I appreciate your feedback and please do let me know if you find more information on this. I would love to hear and learn what you find.