Hard drives in Mac computers are almost always partitioned in a such way that there is only one partition that stores the system and user files. There are two problems when when recovering files from a Mac system disk:

• System Integrity Protection introduced into macOS since OS X El Capitan. It blocks unauthorized programs from deep access to the system disk. Although that indeed protects Mac computers from various malicious software, it also prevents many file recovery programs from doing their jobs - file recovery.
• Program installation which requires large data copy to the disk where the lost files were located. This data may overwrite lost files rendering them unrecoverable.

R-Studio for Mac overcomes these problems. Apple has authorized full access for R-Studio for Mac to the protected disk(s), and it can be started from an external device without copying anything to the system disk. This article explains how to utilize these features for file recovery from the system disk of a Mac system.

Preparation
Use another computer to download and write the installation file to a USB disks. This computer may not necessarily be a Mac computer, it may as well be a Windows or Linux machine. If the USB disk is large enough, you may store recovered files on it. It should be formatted as a FAT32 device, or, if you're going to store files larger than 2 GB, as an exFAT device.

File recovery
1. Connect the USB disk to the Mac computer with the lost files, open it, and double-click the RStudio*.dmg file.

Fig.1. USB disk with R-Studio installation file

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2. The installation window will appear.

Fig.2. R-Studio installation window

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Double-click the R-Studio for Mac icon rather than drag it to the Application folder.

3. When you start R-Studio for the first time a System Extension Blocked warning message will appear.

Fig.3. System Extension Blocked warning message

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At the same time R-Studio will throw its own message:

Fig.4. R-Studio warning message

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If you need access to your system disk, click the Open Security Preferences button on any of the messages, go to the Security and Privacy preferences, and click the Allow button.

Fig.5. Security and Privacy preferences

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You need to follow this procedure only once, next time R-Studio for Mac will start without this message.

4. R-Studio will start with full access to the system disk.

Fig.6. R-Studio with full access to the Mac system disk

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5. Double-click the system disk and go to the files on it.

Fig.7. Files on the Mac system disk

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You may recover files from it as if R-Studio for Mac was installed on the computer.