IMac beeps during installation of El Capitan update

Hello Glenn Millam,

If I understand correctly, you tried to update the OS and the Mac beeped while starting up. You wanted to know more about the Mac startup tones. I would recommend you read this article, it should be helpful in troubleshooting your issue.

About Mac computer startup tones - Apple Support

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Take care

This also happened to me.

I just installed El Capitan on my Late 2009 27" iMac and just before setup finished there was this long beep tone and an EFI update bar.

After the EFI update was completed, my system profiler told me that Boot-ROM is now at version IM111.0034.B04 which is likely two revisions newer then the latest release on the Apple website (...B02).

Now I'm wondering what has been fixed in combination with El Cap...

I have a macbook pro which i have a new hard drive and a ground up install. During the update to el Capitan it did just as you stated yours did. A very weird sounding beeeeep. followed by a black screen. 30 secs later it came to a screen simular to a normal boot up but the graphics were a bit different. The progress bar under the apple was about 4 times thicker then normally it would be. i never intervened in its process.. just let it go. It rebooted on its own and got back to a "give me your password" log in screen. which i provided the password. and then it rebooted and acted like normal start up. 20mins later it rebooted again and gave a normal log on screen. As weird as it was, it seems to have either been normal part of the process, or whatever it didn't like was fixed somehow. either way in my case i did not touch a thing and it eventually brought me back to a normally functioning OS X.

I've had two iMacs (2012) that I recently updated to 10.11 (El Capitan) from much earlier versions - 10.8 and 10.9 I think for either computer.

Upon restart, both made the loud beep, followed by a progress bar. At the end of the progress bar, the computer restarted again and proceeded with a normal Mac OS startup (into 10.11 this time).

So it is a normal part of the process. You do not need to intervene!

One thing that helped me out in both of these upgrades was the fact that I had upgraded to fast SSD drives in both computers, so the down-time was short, meaning I never had time to get worried and hard-restart the computer.

Indeed if I had the previous slow startup of the original drives, I probably would have got worried and done something.

As a previous user mentioned, it seems the sounds and the unusual extra progress bar is a firmware update (aka. EFI), so it boots into a special EFI-updating mode, which apparently emits a loud, cacophonic beep when starting in that mode.

Since these two computers hadn't been updated for a very long time, I guess they had firmware updates to install along with the 10.11 OS install.

How long should I wait? The progress bar seems to have stuck at around 80% of completion... Been like that for about 20 mins now 😟

Wait until tomorrow, and let us know how long it took!

Well.. I waited another 2 hours, and not one extra pixel's worth of progress happened on the bar (I'd even put a sticky note over the already completed section of the bar so I could empirically tell if anything else had happened rather than convincing myself it had done anything) - and nothing... Did a PRAM reset and reboot and it all came up 100% fine.. Mucho oddness