You sit down at your table, boot up your Mac and…surprise. Instead of macOS, what appears to you is a smooth screen with a lightning symbol. And logically you panic, because you think your Mac is broken. Don’t worry: something happened, but it’s not bad news.
The Mac’s “target mode” can do its job
That symbol that appears on your screen is the Thunderbolt connection logo, and in this context it means the Mac is waiting for an external hard drive (or another Mac) connected to this port to boot from it instead of booting using its own internal drive as usual.
What happens is that, for some reason, the Mac has been put into “target mode” to depend on another device when it starts. It could be another Mac, it could be an external drive that has a bootable macOS installed.
Solving it is done simply by turning off the Mac, unplug any external drive you have plugged in and turn it back on. When you do this, also make sure you’re not pressing any keys (“target mode” usually starts when you press the T key on some Macs). When you turn on your Mac, you can also check the “Startup Disk” section in System Preferences, to make sure that the computer’s internal disk is selected and that it’s the only option.
