Mostly, Time Machine will detect a connected unit and would ask you to take its backup automatically. You can use an external data source so that Mac can create a Time Machine on it. If you do not want to let go of your important files during the reinstallation process, then consider taking their backup beforehand. Time Machine is the inbuilt solution to backup data on Mac and restores it afterward. There are different ways to backup data on Mac. If you don't want to suffer from an unexpected loss of your important files during the reinstallation, then you should take a backup of your data beforehand.
How to Reinstall Mac OS? Step 1: Backup Files on Mac Once you are ready, simply take the following steps to reinstall macOS without losing its data.
Startup issues – Apart from the firmware, there would be a problem with your Mac's startup or bootstrap program as well.It can happen when macOS seems to malfunction and can't complete the needed operations. Firmware issues – Mostly, a firmware issue related to macOS demands a reinstallation of the operating system.The following are some of the common problems that force a complete reset on a Mac. There are all kinds of reasons for us to reinstall macOS on the system.
Recover Erased Files after Reinstalling Mac OS.Troubleshooting: How to Reinstall Mac OS?.