If you purchased your Mac with OS Catalina (10.15.x) or Big Sur (11.xx.x) already installed, you can skip the uninstall part above and follow the instructions below.

6 'high level' steps needed, follow down the page to make this a painless systematic process

1.Is your CAC reader 'Mac friendly'?
2.Can your Mac 'see' the reader?
3.Verify which version of Mac OS you have
4.Figure out which CAC (ID card) you have
5.Install the DoD certificates
5a.Additional DoD certificate installation instructions for Firefox users
6.Decide which CAC enabler you want to use (except for 10.12-.15 & 11)
OS X Mountain Lion is available now as a download from the Mac App Store. Just click the Mac App Store icon in your dock, purchase Mountain Lion, and follow the onscreen instructions to install it. Mountain Lion will not only make your Mac work better, it also makes your Mac work even better with your iPhone, iPad, and iPod touch. Because it comes with iCloud, your mail, calendars, contacts. All versions of Mac OS X that were made to run on PowerPC systems (with the exception of Leopard) had a Mac OS 9 emulation layer called 'Classic'. It allowed Mac OS X to run Mac OS 9 applications that weren't updated to run natively on OS X (known as carbonization based on the Carbon API).
Step 1: Is your CAC reader Mac friendly?
Visit the USB Readers page to verify the CAC reader you have is Mac friendly.
Visit the USB-C Readers page to verify the CAC reader you have is Mac friendly.
'Some, not all' CAC readers may need to have a driver installed to make it work.
NOTE: Readers such as: SCR-331 & SCR-3500A may need a firmware update (NO OTHER Readers need firmware updates).
Information about these specific readers are in Step 2
Step 2: Can your Mac 'see' the reader?

Plug the CAC reader into an open USB port before proceeding, give it a few moments to install

Step 2a: Click the Apple Icon in the upper left corner of the desktop, select 'About This Mac'
Step 2b: Click 'System Report..' (button)
Step 2c: Verify the CAC reader shows in Hardware, USB, under USB Device Tree. Different readers will show differently, most readers have no problem in this step. See Step 2c1 for specific reader issues.
Step 2c1: Verify firmware version on your SCR-331, SCR-3310 v2.0, GSR-202, 202V, 203, or SCR-3500a reader. If you have a reader other than these 6, Proceed directly to step 3
Step 2c1a-SCR-331 reader
If your reader does not look like this, go to the next step.
In the 'Hardware' drop down, click 'USB.' On the right side of the screen under 'USB Device Tree' the window will display all hardware plugged into the USB ports on your Mac. Look for “SCRx31 USB Smart Card Reader.” If the Smart Card reader is present, look at 'Version' in the lower right corner of this box: If you have a number below 5.25, you need to update your firmware to 5.25. If you are already at 5.25, your reader is installed on your system, and no further hardware changes are required. You can now Quit System Profiler and continue to Step 3.
Step 2c1b-SCR-3310 v2.0 reader
If your reader does not look like this, go to the next step.
In the 'Hardware' drop down, click 'USB.' On the right side of the screen under 'USB Device Tree' the window will display all hardware plugged into the USB ports on your Mac. Look for “SCR3310 v2.0 USB Smart Card Reader.” If the Smart Card reader is present, look at 'Version' in the lower right corner of this box: If you have a number below 6.02, it will not read the 'G+D FIPS 201 SCE 7.0' CAC on Mac OS 11.xx.x or 10.15.7. I contacted HID (the company that makes these readers) on 14 DEC 2020 to find a way to update the firmware to 6.02. They said there is not firmware update for the reader. If your reader is older, you may need a new one. Please look at: https://militarycac.com/usbreaders.htm to find a compatible one. If you are already at version 6.02, your reader should work fine on your Mac and no further hardware changes are required. You can now Quit System Profiler and continue to Step 3.
Step 2c1c-SCR-3500A reader
If you have the SCR3500A P/N:905430-1 CAC reader,you may need to install this driver, as the one that installs automatically will not work on most Macs. Hold the control key [on your keyboard] when clicking the .pkg file [with your mouse], select [the word] Open
Step 3: Verify which version of MacOS you have?
(You need to know this information for step 6)
Step 3a: Click the Apple Icon in the upper left corner of your desktop and select 'About This Mac'
Step 3b: Look below Mac OS X for: Example: Version 10.X.X, or 11.X
Step 4: Figure out which CAC (ID Card) you have
(You need to know this information for step 6)
Look at the top back of your ID card for these card types. If you have any version other than the seven shown below, you need to visit an ID card office and have it replaced. All CACs [other than these six] were supposed to be replaced prior to 1 October 2012.
Find out how to flip card over video Keep on mac os.
Step 5: Install the DoD certificates (for Safari and Chrome Users)
Go to Keychain Access

Click: Go (top of screen), Utilities, double click Keychain Access.app

(You can also type: keychain access using Spotlight (this is my preferred method))

Select login (under Keychains),and All Items (under Category).

Download the 5 files via links below (you may need to <ctrl> click, select Download Linked File As.. on each link) Save to your downloads folder
Please know.. IF You have any DoD certificates already located in your keychain access, you will need to delete them prior to running the AllCerts.p7b file below.
Mac
https://militarycac.com/maccerts/AllCerts.p7b,
https://militarycac.com/maccerts/RootCert2.cer,
https://militarycac.com/maccerts/RootCert3.cer,
https://militarycac.com/maccerts/RootCert4.cer, and The needlemancer - demo playground #godotwildjam27 mac os.
Double click each of the files to install certificates into the login section of keychain
Select the Kind column, verify the arrow is pointing up, scroll down to certificate, look for all of the following certificates:
DOD EMAIL CA-33 through DOD EMAIL CA-34,
DOD EMAIL CA-39 through DOD EMAIL CA-44,
DOD EMAIL CA-49 through DOD EMAIL CA-52,
DOD EMAIL CA-59,
DOD ID CA-33 through DOD ID CA-34,
DOD ID CA-39 through DOD ID CA-44,
DOD ID CA-49 through DOD ID CA-52,
DOD ID CA-59
DOD ID SW CA-35 through DOD ID SW CA-38,
DOD ID SW CA-45 through DOD ID SW CA-48,
DoD Root CA 2 through DoD Root CA 5,
DOD SW CA-53 through DOD SW CA-58, and
DOD SW CA-60 through DOD SW CA-61
NOTE: If you are missing any of the above certificates, you have 2 choices,
1. Delete all of them, and re-run the 5 files above, or
2. Download the allcerts.zip file and install each of the certificates you are missing individually.
Errors:
Error 100001 Solution
Error 100013 Solution
You may notice some of the certificates will have a red circle with a white X . This means your computer does not trust those certificates
You need to manually trust the DoD Root CA 2, 3, 4, & 5 certificates
Double click each of the DoD Root CA certificates, select the triangle next to Trust, in the When using this certificate: select Always Trust, repeat until all 4 do not have the red circle with a white X.
You may be prompted to enter computer password when you close the window
Once you select Always Trust, your icon will have a light blue circle with a white + on it.
The 'bad certs' that have caused problems for Windows users may show up in the keychain access section on some Macs. These need to be deleted / moved to trash.
The DoD Root CA 2 & 3 you are removing has a light blue frame, leave the yellow frame version. The icons may or may not have a red circle with the white x
or DoD Interoperability Root CA 1 or CA 2 certificate
DoD Root CA 2 or 3 (light blue frame ONLY) certificate
or Federal Bridge CA 2016 or 2013 certificate
or Federal Common Policy CAcertificate
or or SHA-1 Federal Root CA G2 certificate
or US DoD CCEB Interoperability Root CA 1 certificate
If you have tried accessing CAC enabled sites prior to following these instructions, please go through this page before proceeding
Power putt mac os. Clearing the keychain (opens a new page)
Please come back to this page to continue installation instructions.
Step 5a: DoD certificate installation instructions for Firefox users
NOTE: Firefox will not work on Catalina (10.15.x), or last 4 versions of Mac OS if using the native Apple smartcard ability
Download AllCerts.zip, [remember where you save it].
double click the allcerts.zip file (it'll automatically extract into a new folder)
Option 1 to install the certificates (semi automated):
From inside the AllCerts extracted folder, select all of the certificates
<control> click (or Right click) the selected certificates, select Open With, Other..
In the Enable (selection box), change to All Applications
Select Firefox, then Open
You will see several dozen browser tabs open up, let it open as many as it wants.
You will eventually start seeing either of the 2 messages shown next
If the certificate is not already in Firefox, a window will pop up stating 'You have been asked to trust a new Certificate Authority (CA).'
Check all three boxes to allow the certificate to: identify websites, identify email users, and identify software developers
or
'Alert This certificate is already installed as a certificate authority.' Click OK
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Once you've added all of the certificates..
• Click Firefox (word) (upper left of your screen)
• Preferences
• Advanced (tab)
• Press Network under the Advanced Tab
• In the Cached Web Content section, click Clear Now (button).
• Quit Firefox and restart it
Option 2 to install the certificates (very tedious manual):
Click Firefox (word) (upper left of your screen)
Preferences
Advanced (tab on left side of screen)
Certificates (tab)
View Certificates (button)
Authorities (tab)
Import (button)
Browse to the DoD certificates (AllCerts) extracted folder you downloaded and extracted above.
Note: You have to do this step for every single certificate
Note2: If the certificate is already in Firefox, a window will pop up stating: 'Alert This certificate is already installed as a certificate authority (CA).' Click OK
Note3: If the certificate is not already in Firefox, a window will pop up stating 'You have been asked to trust a new Certificate Authority (CA).'
Check all three boxes to allow the certificate to: identify websites, identify email users, and identify software developers

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Once you've added all of the certificates..
• Click Firefox (word) (upper left of your screen)
• Preferences
• Advanced (tab)
• Press Network under the Advanced Tab
• In the Cached Web Content section, click Clear Now (button).
• Quit Firefox and restart it
Step 6: Decide which CAC enabler you can / want to use
Only for Mac El Capitan (10.11.x or older)
After installing the CAC enabler, restart the computer and go to a CAC enabled website
NOTE: Mac OS Sierra (10.12.x), High Sierra (10.13.x), Mojave (10.14.x), Catalina (10.15.x), and Big Sur (11.1) computers no longer need a CAC Enabler.
Try to access the CAC enabled site you need to access now
Mac support provided by: Michael Danberry
Mac OS X El Capitan: Being the 12th major release of macOS, Mac OS X El Capitan belongs to the Macintosh-Unix family of OS. It was majorly released for server OS for Macintosh computers and Apple Inc.’s desktop. Being the successor of OS X Yosemite, it mainly focuses on stability, performance, and security. There is a history in its name. Mac OS El Capitan was given its name behind a type of rock formation that is found in the Yosemite National Park, focusing its predefined goals towards a refined edition of Yosemite. It was the final version that was released beneath the name OS X, its successor, Sierra, was given the name under Mac OS X due to the reason that OS X is now named Mac OS.
License
Official Installer
File Size
5.7GB
Language
English
Developer
Apple Inc.

Mac OS El Capitan was released to manufacturing on 20th September 2015, almost three and a half years ago. Its latest version 10.11.6 (15G22010) was released on 9th July 2018, almost 7 months ago. It runs on the platform including x86-64. Mac OS El Capitan is preceded by OS X 10.10 Yosemite and it is succeeded by macOS 10.12 Sierra.

System requirements

All Macintosh PCs that can run Mountain Lion, Mavericks, or Yosemite can run El Capitan in spite of the fact that not the majority of its highlights will take a shot at more seasoned PCs. For instance, Apple noticed that the recently accessible Metal API is accessible on “all Macs since 2012”.
The following computers can run mac OS el Capitan provided that they must have a ram of 2GB or more –
Of these PCs, the accompanying models were outfitted with 1GB RAM as the standard alternative on the base model when they were sent initially. They can possibly run OS X El Capitan in the event that they have at any rate 2GB of RAM-
The accompanying PCs bolster highlights, for example, Handoff, Instant Hotspot, AirDrop between Mac PCs and iOS gadgets, just as the new Metal API:
The redesign differs in size contingent on which Apple Mac PC it is being introduced on, in many situations, it will require around 6 GB of plate space.

What is DMG File

DMG represents Disk picture document. It is much the same as the ISO record which keeps running on Windows yet it keeps running on Mac OS. DMG is area by a segment of optical stockpiling media. DMG record is utilized rather than an optical plate. It has .dmg augmentation. Past Disk Image File has .smi and .img augmentation which depends on New Disk Image Format (NDF). The present DMG document depends on Universal Disk Image Format (UDIF). Widespread Disk Image Format underpins Zlib, ADC, bzip2 pressure calculation. DMG record depends on figuring out however it’s documentation isn’t discharged by Apple Company. It is more secure than ISO record. Plate Image Files are distributed with a MIME kind of utilization which is known as x – apple circle picture. There are distinctive record frameworks inside circle picture.

What is ISO file

ISO document is the copy or file record that contains every one of the information of optical plate-like CD or DVD. It represents the International Standard Organization. It is utilized to duplicate the precise record in light of the fact that in typical duplicate, it needs header document yet in an ISO record, every one of the information including the header record is The ISO name is taken from ISO9660 on the grounds that this component is utilized in circles yet ISO record has UDF(ISO/IEC 13346) document framework is utilized which is generally utilized in DVDs and Blu-beam Disks. It accompanies .iso augmentation.

Installing Mac OS X El Capitan

The installation process of mac os el Capitan includes two methods. The first method is termed as an upgrade install, also known as the default method. The second method is known as a clean install.
Upgrade install-
Follow to below steps-

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Clean install-
For a clean install, at first, you will need a bootable version of OS X El Capitan installer. After getting a bootable drive or device follow the steps to boot-
  1. Insert the device containing os X El Capitan installer.
  2. While holding down the option key, restart the system
  3. Select that particular device which contains the installer file.
  4. The system will start from the device containing the installer.
  5. Now, to erase the older version, click on disk utility and then click continue.
  6. After the start of disk utility, select the volume to erase and perform the further steps to format the selected drive.
Starting the installation process-
  1. With the startup volume deleted, you’re currently prepared to start the establishment of OS X El Capitan.
  2. In the OS X utility window, select Install OS X, and snap the Continue catch. The installer will begin, despite the fact that it might take a couple of minutes.

Performing clean installation

  1. In the install os window, click the continue button.
  2. Read the terms and condition and agree with them.
  3. Select the target disk and click on the install option.
  4. Type your administrator password and click OK.
  5. The installer will duplicate the needed files and then restart.
  6. The progress bar will show the time required.
  7. After the installation, the system will restart. Follow the further guided steps to setup mac OS X El Capitan.

Mac OS X El Capitan setup process-

  1. The Welcome screen shows, requesting that you select which nation your Mac will be utilized in. Make your choice from the rundown, and snap the Continue catch.
  2. Make your keyboard layout selection and press continue.
  3. The Transfer Information to This Mac window will show up. Here you can move existing information from a Mac, PC, or Time Machine reinforcement to the clean introduce of OS X El Capitan.
  4. Enable Location Services- By enabling this service, you are allowing the app to see the geographic location of your Mac.
  5. Select the further option from the drop-down menu.
  6. It’s up to you whether you have to enter an Apple Id or create a new one in the first step.
  7. After that, read the agreement and agree to it.
  8. Create a Computer Account choice will show. This is the head account, so make certain to take note of the username and password word you chose.
  9. The Select Your Time Zone window will show. You can choose your time zone by tapping on the world guide or pick the nearest city from a rundown of real urban areas around the globe. Make your choice, and snap Continue.
  10. The Diagnostics and Usage window will inquire as to whether you wish to send data to Apple and its designers about issues that may happen with your Mac or its applications. Make your determination, and snap Continue

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The setup procedure is finished. After a couple of minutes, you’ll see the OS X El Capitan work area, which implies you’re prepared to begin investigating the perfect establishment of your new OS.