If your Mac is having problems, macOS has multiple keyboard combinations that you can hold to recover your system, start in safe mode, run diagnostics, and more. Here's how to use them.

Once you've entered macOS Recovery, you can repair your Mac's internal drive, reinstall macOS, restore your data from a Time Machine backup, start up in safe mode, transfer files between two Macs ...

If the OS uses the first 700 MB when booting normally, but only the first 500 MB when booting in safe mode, it will appear to be broken in normal mode, and fine in safe mode.