Summary: In this article, we'll share the step-by-step process for using macOS native tools such as Disk Utility, Time Machine and macOS Recovery to repair and restore macOS Catalina 10.15 start-up disk. Additionally, we speak about availing the free trial version of Stellar Data Recovery Professional software which fully supports macOS Catalina. Internet Recovery has a number of useful applications. This guide will show you how to use Internet Recovery to install a fresh copy of macOS to a blank or corrupted SSD. Internet Recovery is only available on Apple computers made after 2009.

Customers get Software upgrades for Mac computers through the online app store ever since the release of macOS Lion. Be it a clean install or upgrade, Mac App store require you connect to the internet to download whole installation files. The online OS update is OK if you have stable broadband connectivity and enough bandwidth. Apple also provides an option for internet recovery of macOS if your computer fails to boot. But there are situations where you can’t download the OS from the Apple servers. Having a macOS offline installer or a bootable USB disk is the only solution in such scenarios.

Download macOS offline Installer

Installing macOS without internet requires a bootable USB disk. And you need to download a full-size macOS installer for making this disk. Hence, before anything, we will tell you how to download macOS offline installer directly from Apple to your Mac using a GitHub script.

Download the gibMacOS script from this link and extract it to your Mac. From the extracted folder, run the script called gibMacOS.command — it will open the Terminal as shown below.

Wait for the script to fetch the macOS download catalogue from Apple. When the list is ready choose a number as per your OS option and press the Return/Enter key. You can use this script to download macOS Big Sur (11.1), Catalina (10.15), Mojave (10.14), and High Sierra (10.13.6). A new folder “macOS downloads” appears in the “gibMacOS-master” as you provide a download option, and the OS starts downloading as you can see in the video.

The OS gets downloaded as parts, PKGs, DMGs and other files. For macOS versions up-to Big Sur, you need to use another script “BuildmacOSInstallApp.command” in the main folder to join all the downloads and make the full macOS installer app. All you need is to drag and drop the downloaded macOS folder to the terminal window. The script will build the offline installer and save it to the same folder.

You don’t need to use the second script if your choice is macOS Big Sur. Because, for macOS Big Sur you get a full-size installer from Apple in PKG format. Double-click and open the InstallAssistant.PKG to save the “Install macOS Big Sur.app” to the Applications folder.

Apart from this, some older versions of macOS are available for direct download from Apple (Sierra, EL Capitan, Yosemite) in DMG format.

How to make a macOS bootable USB installer

It is possible to create a backup macOS installer on a DVD or USB drive but later is the best choice. Most of the Macs these days don’t have DVD slots, and the installation through a disc is very slow. Hence we discuss only about making macOS recovery installer on a USB disc. For this you need a Mac and a USB pen drive of at least 8 GB space, and a third-party app called DiskMaker.

1. First, download a copy of Disk Maker from the official website.
2. Choose a version as per your requirement. For example, if you want make USB installer for macOS Catalina 10.15, download Disk Maker X 9.
3. I use an MacBook Pro with High Sierra. Hence, I’m downloading disk maker X 7.
4. The next step is to download full macOS installer from the App Store to your Applications folder.
5. Run Disk Maker DMG installer and copy the app to the Applications.
6. The application will automatically detect the downloaded macOS as shown in the screenshot. Select “Use this Copy” if you would like to proceed with it.
7. Next is setting the USB pen drive for creating the bootable installer. Choose the third option ” An 8 GB USB Thumb drive.”
8. The app will list all drives in your system. Pick the one you would like use as macOS USB installer.
9. Ignore the warning message and proceed to Erase and Create macOS USB disk. Don’t forget to provide your admin password when it prompts.
10. Keep checking notifications tab for progress. When it completes you will find Install macOS Mojave (or High Sierra) mounted to your desktop as a USB disk.
11. That’s your backup. Unmount and keep the USB installer to a safe place.

Unfortunately, the above app is not compatible with macOS Big Sur 11 and above. Check the instructions on Apple website, the manual method to make macOS offline USB installer.

What is the alternative if I don’t have usable Mac at the moment?

This method is applicable when you have a Mac, and the internet recovery fails. But think about a situation when your only Mac stops to boot, and you don’t have USB installer or TimeMachine backup! Downloading MacOS DMG file might save you. If you have a macOS DMG, you can create bootable USB disk from a Windows PC — use a software like TransMac for this purpose.

Clean installing or restoring a Mac offline with USB pen drive

As said earlier, you could use the macOS USB installer to do a clean install or restore it upon a boot failure. Just connect the USB disk and press the Option/Alt Key when you hear the boot sound. The Mac will then display all the startup disks available.

Pick the USB drive starting with the label “Install macOS” and follow on-screen instructions to complete the installation. If you need, use the Disk Utility to format the drive for doing a clean install.

Reinstalling the OS of your Mac is sometimes required as a last resort option when troubleshooting a certain software problem. If none of the other fixing methods have worked out so far, this final fixing method should be enough to bring things back to normal and restore your Mac to its regular state.

However, before you continue with the macOS reinstallation, we advise you to make sure that you have indeed exhausted all other options and made sure that this really is the only thing that you haven’t tried so far. If that is your case and you are sure you wish to continue, here is a guide that can help you with the reinstallation process.

Accessing macOS Big Sur Recovery

MacOS Recovery is a special mode that gives you access to different system repair tools that can help you deal with different issues that your Mac has. In the macOS Recovery Mode, you can use the Time Machine backup tool, get online help for your problem, erase the hard drive of your Mac, reinstall your macOS, and more.

Entering Recovery Mode is a bit different depending on whether your Mac has an Apple Silicon or an Intel processor. To find out the type of processor that your Mac has, click on the Apple Menu, select About this Mac, and look at the information in the window that opens. If you see an entry labeled Chip with the model of the chip written after it, this means that your Mac is with an Apple Silicon processor. If instead of that there is a “Processor” entry followed by the specific processor model, it means that your Mac is equipped with an Intel processor. Depending on which one of the these two your Mac has, do either of the following:

• Macs with Apple Silicon processors: With the computer turned off, press and hold down the Power Button, wait for the Startup Options screen to get displayed on your screen, and then release the button. Now select the Options icon and click on Continue.

• Macs with Intel processors: Make sure that your Mac is connected to the Internet and turn it off. Then start the computer and immediately after that hold down the Command ⌘ + R keys from the keyboard. Keep holding them until the Apple logo (or another image) gets displayed on the screen.

If your Mac requires you to log in to any of your Mac accounts, select an account you know the password for and that has Admin privileges.

Reinstalling macOS Big Sur from Recovery Mode

Now that you have entered Recovery Mode on your Mac, it is time to reinstall the OS of the computer.

1. From the Utilities screen of the macOS Recovery, select the Reinstall option and click on Continue.
2. Follow the on-screen instructions and steps – they will guide you through the whole process.
3. Be sure to adhere to the following tips while in the process of reinstalling macOS.
   • Do not put your Mac to sleep or close its lid while the reinstallation process is underway. While macOS is being reinstalled, your Mac may restart several times and the screen may go black and stay that way for several minutes – even if this happens, stay patient and wait for the process to complete. Until macOS gets reinstalled, do not close, put to sleep, or turn off your MacBook!
   • If you are asked to unlock the disk of your Mac, do so by filling in the password that you use to log in to your main Mac account.
   • If the macOS installer cannot find your disks or it tells you that the installation cannot be completed on the specified disk, you may first need to erase that disk and try again.
   • If you are given the choice between installing Macintosh HD or Macintosh HD Data, pick the first option.

Once the installation of macOS is complete, you will be asked to set up the preferences of the computer. Complete the set up according to your personal preferences and you will be good to go.

If you have reinstalled the OS because you intend to sell or give away your Mac and want to allow its new owner to set it up themselves, press Command-Q to close the Setup Assistant and leave the setting-up process to the future owner of the computer. After that, click on the Shut Down button to turn off the Mac.

Additional options and tips for installing/reinstalling macOS

Os X Catalina Internet Recovery

The default setting of macOS Recovery is that it will install the latest version of the operating system that was previously installed on the computer. If you want, you can get it to install a different version of the OS by using one of the following options:

How To Install Catalina From Internet Recovery

• On a Mac with an Intel processing unit, you can press down and hold the Option + Command + R keyboard combination (instead of the Command ⌘ + R combination) as soon as you turn on the computer. This will cause macOS Recovery to install the latest stable macOS version that is compatible with the machine. There are two exceptions here:
  • If macOS Sierra 10.12.4 or another later version of macOS has never before been installed on the computer, the macOS version that will get installed will be the one that originally came with the machine, or another macOS version that is closest to the original one.
  • If the computer is equipped with the Apple T2 Security Chip and a macOS update has never before been installed on the machine, macOS Recovery will install the latest version of the OS that was installed on the computer.
• If your Mac previously had the macOS Sierra 10.12.4 version of the OS or a later version, you can also use the Shift + Option + Command + R combination once you start the computer (press and hold the mentioned buttons) instead of the two previous combinations to make macOS Recovery install the OS version that was originally on your Mac when you bought it or the closest other OS version that is still available.
• To install a different macOS version that macOS Recovery doesn’t allow you to install, you can instead go to the App Store and install the desired OS version from there.
• Another way to reinstall your macOS is to create a bootable USB drive and install the desired OS version from it.
• If the logic board of your Mac has just been replaced after your machine has been repaired by a technician, it is likely that macOS Recovery will only offer you to install the latest OS version that is compatible with the computer.
• If the entire disk of your Mac has just been erased (instead of only the startup volume), macOS Recovery will probably only allow you to install the version of the OS that came with the computer when you first bought it (or the closest other available macOS version).