If you're having problems with a disk, Disk Utility can check the disk and repair problems it detects. How to repair a disk To explore the Disk Utility User Guide, click Table of Contents at the top of the page, or enter a word or phrase in the search field. First, repair your disk drive with Disk Utility: Repair the Drive for El Capitan or Later. Restart the computer and after the chime hold down the Command and R keys until the Utility Menu appears. Select Disk Utility and press the Continue button. Then select the indented (usually, Macintosh HD) volume entry from the side list.
The macOS Mojave installer software is just over 6GB, so you need a USB external drive that can hold that much data. The drive can be a thumb drive, hard drive, or SSD. If you want to use a thumb.
Scroll down and open the ‘Utilities' folder, and double-click to open Disk Utility. Select your ‘Macintosh HD' hard drive from the left drawer if not select already. Click on the ‘Partition' tab option present up-top next to ‘First Aid.' Click on Partition and not ‘Add Volume.'.
Mojave Disk Utility
There is no longer any need to repair permissions when using Mojave.