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My iMac (late 2015, 8/1TBSSD) has run happily for more than a year on High Sierra, and I just now upgraded the OS to Mojave. Now, the machine won't boot into safe mode or into Recovery. When I hold down the appropriate keys before or at the startup chimes, the regular boot ensues. Booting is brisk and correct. No firmware password is set. The keyboard is a wired Apple keyboard A1243; in use it gives no indication of fault with any of its keys.

In all other respects, the Mac operates as it should. I updated my MacBook Air (mid-2013, 4/512SSD) at the same time using the same Mojave installer, and the Air behaves as it should.

What's gone awry here, and how might I return the iMac to its correct functioning?

  • Have you attempted to turn off the machine during the boot process, and then restart, holding the appropriate keys down? – Matthew Barclay May 1 at 2:11
  • @MatthewBarclay I haven't, never heard of that technique. You mean press and hold the power button down (or unplug the machine) while the booting progress bar under the Apple symbol is moving to the right? – David May 1 at 2:44
  • that's generally the technique that I use anymore to boot into recovery or bootcamp, as my computer (macMini 2014) doesn't give me enough time otherwise. I have no clue whether that would work on iMac – Matthew Barclay May 1 at 20:42
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I am relieved (and embarrassed) to report that the fault was a poor USB connection between the wired keyboard and the iMac USB port. A little unplugging and re-plugging solved everything. The iMac now boots correctly, in whatever mode I select.

The keyboard connection was only faulty in the computer's un-booted state. When booted, the computer accepted keyboard input from every key, including the various function keys.

The USB connections had been in place for about a year, since I bought the computer. Apparently prudence dictates that these connections be periodically unplugged and reconnected.

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