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Step 2 When the Mac is starting, you need to press and hold CDM + R keys to enter the recovery mode. Step 3 In the Recovery mode, click Disk Utility and erase the Macintosh HD volume. This will delete all your data on your Mac so make sure you have backed up your important data. Step 4 After erasing your disk, you can reinstall macOS. Reinstall macOS. Select Reinstall macOS from the utilities window in macOS Recovery, then click Continue and follow the onscreen instructions. Follow these guidelines during installation: If the installer asks to unlock your disk, enter the password you use to log in to your Mac. If the installer doesn't see your disk, or it says that it can't. Can’t provide any download links here. InstallESB.dmg is part of OS X Lion installation file. Apple lawyers might kick my ass if I provide a download link here. Mac OS X Lion a great operating system try to get it though the legal method.

Here’s how you can create it. Make sure the USB drive is at least 8GB in size, and nothing else is stored on it. Download the macOS Catalina installation file using this link. Turn off the Mac and then insert the bootable USB drive. Now turn the Mac back on while holding the Option key. It will take you to the boot menu where you can select. MacOS Recovery is part of the built-in recovery system of your Mac. It can help you restore data from Time Machine, reinstall the operating system, get online help online, repair or erase a hard disk, and much more.

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.

If you have installed Big Sur, Catalina, Mojave or earlier versions, you may have to reinstall macOS for following reasons:

• Reinstall When There is New macOS Version Available

Apple keeps working constantly to fix bugs, make performance tweaks, add new features or enhance coding. Therefore, doubtless, there will be new versions of macOS available to upgrade and reinstall.

• Your Mac is Running Slow

As we all know, for no specific reason, system reinstallation can magically solve a slow Mac in most cases.

• Your System Keeps Crashing or Works Improperly

When you continuously see error messages appear on your mac, or your programs randomly crash/freeze for no reason, like FaceTime won't work, Contacts or Calendar shows a delay or mess, blue teeth or WiFi won't connect…Then, you have a good reason to reinstall macOS.

• You are Going to Sell the Mac

In the case that you want to sell your mac, besides erasing all your personal data and traces on the mac, you will need to reinstall macOS as well.

It is not complicate to reinstall macOS Big Sur or Catalina, but if you want to reinstall macOS without losing data, there are 3 steps you must follow.

Recover Files after macOS Reinstall

• Recover data lost due to macOS reinstallation, upgrade, downgrade
• Recover data lost due to accident deletion, formatting, etc.
• Restore data from both internal and external storage device
• Recover videos, audios, images, documents, and other 200+ files
• 5 recovery modes to find files back easily
• High recovery rate
• Support Big Sur, Catalina, Mojave and earlier

3 Steps to Reinstall macOS Big Sur or Catalina without Losing Data

We all save tons of data on our Mac, so when we decide to reinstall macOS Big Sur/Catalina, the top concern always goes to “will I lost everything if I reinstall macOS”. In fact, reinstallation of macOS doesn’t necessarily cause lost data, it just create a new copy, your existing files and data saved in programs won't be altered or deleted. But just in case of bad luck, we need to do some work on BACKUP, this is crucial for macOS reinstallation without losing data.

Step 1. Prepare Your Mac for Reinstallation.

• Make enough room for Big Sur or Catalina reinstallation, at least 36GB, so the reinstallation process won't pause or stopped for insufficient space.
• Also, quit all apps or programs under work, so your Mac is fully geared to reinstall.
• Check drive conditions. Open Disk Utility and perform the Frist Aid on your hard drive where to reinstall the macOS to make sure your drive is in good condition for reinstallation.

Step 2. Backup All Your Files for macOS Install (Crucial)

Backup is an indispensable step involved in the macOS reinstallation, here are several options to backup your data.

Option One: Using Time Machine

1. Go to Finder>Application, launch Time Machine and choose “Set Up Time Machine”.
2. Click “Select Backup Disk” to choose an external hard drive to back up the files.
3. Then Check the box before “Back Up Automatically”. Also, you can adjust the backup setting in menu “Options”.

If this is the first time you use Time Machine to backup, wait patiently for Time Machine to complete the backup, it will prompt the notification once finishes.

Option Two: Using Hard Drive

1. Connect your hard drive to Mac.
2. Open Finder to check if your hard drive is present under “Devices”.
3. Create a new folder, copy and paste or directly move the items you want to save from Mac to this folder.
4. Finally, eject your hard drive.

Option Three: Using iCloud Service (Backup Desk and Documents Folders)

Reinstall Mac Os Catalina

1. Go to Finder>System Preference, click on “iCloud” to bring up its main interface.
2. Click the “Options” button for “iCloud”, and check the box before “Desktop and Documents Folders”, then click on “Done”.

Most of our mac users prefer to backup all files but apps. So, to save you from troubles of lost data due to macOS reinstallation, you are recommended to keep records of what apps you have installed, the account and password, also, you can take screenshots of the settings.

Step 3. Reinstall macOS Big Sur or Catalina without Losing Data.

Option #1: Reinstall macOS without Losing Data From Internet Recovery

1. Click the Apple icon>Restart.
2. Hold down the key combination: Command+R, you will see the Apple logo.
3. Then choose “Reinstall macOS Big Sur” from utilities window and click “Continue”.
4. Select your hard drive, click “Install” and wait for end of reinstallation.

Option #2 Reinstall macOS without Losing Data From USB

1. Download the macOS Big Sur install from Mac App Store onto your Mac;
2. Then connect the USB flash drive to your Mac;
3. Open Disk Utility program on your Mac, choose the USB flash drive and click Erase to have a clean drive for the reinstallation;
4. Open Terminal, copy and paste sudo /Applications/Install macOS Big Sur.app/Contents/Resources/createinstallmedia;
5. Then add the volume of usb flash drive: --volume /Volumes/MyVolume, replace MyVolume with your usb flash drive name, mine is Untitled;
6. Press Enter, input password and wait for the process to finish;
7. Quit Terminal, restart your Mac, hold Command+R when starting Mac until you see the apple logo;
8. Login into your Mac with password, go to the menu bar>Utilities>Startup Security Utility, enter password again;
9. Choose Medium Security under Secure Boot and allow booting from external media;
10. Restart your Mac, in the same time, hold the Option key;
11. Choose Install macOS Big Sur, click Next and input password, choose Install macOS, continue to finish mac reinstallation from usb.

Mac Won't Turn on after Update to macOS Big Sur?

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How To Reinstall Macos Catalina From Scratch

What if You Lost Data After macOS Big Sur Reinstallation?

However, losing data after reinstallation still happens. It may result from a interrupted installation (power-off/poor internet connection), corrupt setup, insufficient space or improper actions. Then, what to do if you lost data after reinstallation? Here are 2 methods.

Method 1: Use Cisdem Data Recovery to Recover Data

In the case you didn’t do backup before reinstallation, you will need a dedication data recovery program to find the lost data for you.

Here we recommend Cisdem Data Recovery, a powerful mac program allowing users to recover lost/deleted/corrupted/formatted files from a wide range of external or internal storage devices, no matter the file is lost due to human errors, power-off, reinstallation, upgrade, virus attack or disk crash.

Main Features of Cisdem Data Recovery

• Recover files lost due to OS reinstallation, upgrade, downgrade;
• Recover deleted, formatted and lost files;
• Restore files from internal and external hard drive, USB, SD Card, flash drive, etc.;
• Restore videos, audios, images, documents, archives, 200+ types;
• 5 recovery modes: Basic, External Drive, Formatted Drive, Trash, Advanced;
• Preview files before recovery;
• Fast scanning and recovery;

Steps to Recover Lost Data after MacOS Reinstallation

1. Download and install Cisdem Data Recovery on Mac.
   Free Download
2. Choose a Recovery Mode according to your need.
   If you want an efficient recovery, choose “Basic Data Recovery”, if you want to recover files with a higher success rate, try “Advanced Data Recovery”.
3. Select the hard drive where you originally stored the files on mac. Then click “Scan”.
4. Check and preview files found by Cisdem Data Recovery.
5. Select files to recover.
   Check the box before files that you want to recover, then click “Recover” to find back the lost data caused by reinstallation.

Method 2: Use Time Machine to Recover Data with Backup

If you have backed up your files on mac, you can use Time Machine to restore the lost data.

• Step1. Go to Finder>Applications>Time Machine, launch it and choose “Enter Time Machine”.
• Step2. In the popped-up window, use the arrows and timeline to browse the local snapshots and backups.
• Step3. Find the deleted files, then click “Restore” to recover the lost data caused by reinstallation.

Conclusion

The key to reinstall mac OS Big Sur, Catalina or Mojave without losing data is the backup, since no one can guarantee all data will be perfectly maintained after macOS reinstallation. However, if we unfortunately lost files after macOS reinstallation, Time Machine or Cisdem Data Recovery is helpful to recover them back.

So, do you have any other tips to reinstall macOS without losing data? Please share with more of our mac users.