Q: iMac will boot in safe mode but get vertical lines in normal mode

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Unknown.

Some features aren't available in safe mode

Some features of your Mac aren't available when it's started in safe mode, so after you're done using safe mode you should restart your computer normally.

For example, these features don't work in safe mode:

• You can't play movies in DVD Player.
• You can't capture video in iMovie and some other video apps.
• Some audio input or output devices might not work.
• Some USB, FireWire and Thunderbolt devices might not be available.
• Wi-Fi networking might be limited or unavailable depending on the Mac and OS X version you're using.

Safe mode in OS X v10.5 or later also disables accelerated graphics. This makes the OS X menu bar appear solid even if "Translucent Menu Bar" is selected in System Preferences. Your screen might also "blink" or "tear" during login when your Mac is started in safe mode.

Try safe mode if your Mac doesn't finish starting up - Apple Support