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I can't believe no one has ever asked this, but I can't seem to find it:

How long should you hold down key combinations, such as Command + S, while your Mac is booting? I can get this iMac to boot into safe mode (sometimes), but not Single User, Verbose, or Diagnostic modes, which seems strange to me, thus I assume I'm not doing something correctly.

Do you just hold the keys briefly or hold the keys for longer? If the latter, how long exactly? This iMac just has that gray boot screen, and there's no indication that anything is happening, so I have no way of knowing when I should stop pressing the key combos or if pressing them is even doing anything.

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In general, you should hold the key(s) down until they take effect. For single-user and verbose modes, that means hold them until you see text start to appear. For the others, it's generally ok to let go once you see the Apple in the middle of the screen (the Apple is drawn by the booter, which loads after the firmware has detected startup options).

If you have FileVault 2 (full volume encryption) set up, things are a little more complicated, since some key combinations (mainly single-user, verbose, and safe boot) don't take effect until after you've authenticated to unlock the startup volume. It currently seems to do a good job of detecting the key combos before the authentication screen, then having them take effect after you "log in". But if that's not working for you, try holding them down as you press return on the FV2 authentication screen, and keep holding them until you see them take effect.

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That's because it will differ slightly depending on what hardware and the startup behavior it's being preformed on. From Startup key combinations for Mac it states:

Some Mac features are available by holding down certain keys during startup.

Press and hold the keys immediately after you turn on your Mac and hear the startup sound. Keep holding until the described behavior occurs. These combinations work with Intel-based Mac computers.

So the timings will very a little bit although it should be relatively quickly to realize the target behavior it taking place.

As an example my 7 year old MacBook Pro does everything considerable slower then my 2 year old MacBook Pro so to give explicit timings for various startup behaviors will vary as much as there are different speeds of processors, etc. and is why Apple states it in the terms that do and not not in seconds, etc.

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Make sure you hold down the special key(s) upon hitting the power button, and keep holding the special key(s) down until you hear the startup chime.

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  • By "upon" you mean, begin holding down the keys right as you press the power button, correct? – Andy Forceno Sep 2 '15 at 0:33
  • @Sophiejane, It's as it's stated by Apple: "Press and hold the keys immediately after you turn on your Mac and hear the startup sound. " – user3439894 Sep 2 '15 at 0:35
  • That may not be long enough -- the firmware chimes before it checks for startup modifier keys. – Gordon Davisson Sep 2 '15 at 4:15
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It used to be simple - If the screen gets drawn with a total grey background, you can let go since the time to have the startup keys modify the boot order has passed.

Now it's complicated with Boot EFI, Recovery HD and FileVault.

Here is the article from Apple documenting the Mac Hardware and OS X startup sequence.

https://support.apple.com/kb/HT204156

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  • Thanks for the edit @user3439894 - I strongly prefer letting people know I'm sending them to Apple's support site. If you want to edit in the title of the document - that's fine, but please leave the URL visible. Thanks! – bmike♦ Sep 2 '15 at 0:35
  • Then please consider including the name of the the document to which the link is pointing to as well. I'm not the only one that edits links that get posted like you post it. Additionally while you may personally prefer bare links, this community is designed for it users to make improvements to both questions and answers that others post, so you may need to be a bit more accepting of standards and let go a bit of personal quirks. Also the hyperlink shows the URL Address in the Status Bar of the Browser, so nothing is really being hidden from the users. – user3439894 Sep 2 '15 at 0:47

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