This morning my wife’s Macbook Air (maybe 5-6 years old) booted to a failed installation screen. It is running High Sierra with the most recent update. Booting with the Option key showed it was trying to boot from an installation partition. Selecting the hard drive allowed it to boot normally. I will attempt to sort this out when I get home tonight so would appreciate any suggestions. I suspect I will end up doing a complete reinstall but maybe this is a simple fix.
All help much appreciated.