if i reinstall Mac OS will I loose all the data on the hard drive?

macOS is designed to preserve your data when reinstalling if you don't wipe the disk and simple start reinstalling, but this is a very serious process and you shouldn't trust it 100%.

Reinstalling and operating system makes deep changes to the entire machine (drivers for CPUs, GPUs, drives, displays, I/O ports, etc.) and that can mess with any data preset in the disk.

That's why I strongly advise you to backup to an external drive and everything will be fine.

Not unless a problem occurs. Back it up first.

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