Help! Internal hard disk won't unmount to be erased!
My internal hard drive will not erase because disk will not unmount. Trying to erase and reinstall OS X system.
MacBook Pro, macOS High Sierra (10.13)
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Hi,
if you booted from the internal hard disk, then it's in use and can't be unmounted because it contains the root file system. You need to boot from the internet, then start disk utility to erase the internal disk.
Be sure to have a full backup before erasing your hard disk if you need to restore data later on!
Best regards,
E.