iMac not recognizing my external monitor after I updated to Catalina 10.15.7

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iMac not recognizing my external monitor after I updated to Catalina 10.15.7

I have an iMac (21.5-inch, late 2013) with an Acer external monitor connected to it. The connecting cable is a USB 3.0 to HDMI.

Very happy with this setup and zero problems with it for many years. Until yesterday, when I updated the iMac's software to Catalina 10.15.7.

Now, the Acer monitor is a black screen that flashes the words "no signal."

If I go to my iMac's System prefs -> Displays and click "detect displays" nothing happens.

I tried resetting NVRAM. Also resetting SMC. (These were suggested steps I pulled off of a Google search.)

This is very disappointing. How do I get the use of my external monitor back, please?

iMac 21.5″, macOS 10.15

Posted on Nov 16, 2021 11:46 AM

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Answer 1

Best reply

jbmcnabb Author

Level 1

16 points

Nov 17, 2021 2:22 PM in response to rkaufmann87

I so appreciate your help with this, rkaufmann87. The good news -- I got the external monitor up and working again today. How I solved it (with help from apple support chat) -- the company that makes the HDMI to USB 3.0 adapter I'm using to has a driver for macOS 10.15. So eventually, once I found that, I was able to download and install the driver, and in a few more clicks, my monitor and iMac were paired up and working together again.

Thank you again for your help!

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Answer 2

rkaufmann87

Level 10

141,024 points

Nov 16, 2021 11:58 AM in response to jbmcnabb

Well you can always try restarting in Safe Mode ( Safe Mode Reset ) and let us know if that works. If it does not please verify the connections and then begin troubleshooting the connections.

BTW, Another reset to try is resetting the NVRAM ( NVRAM Reset )

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Answer 3

Nov 16, 2021 12:01 PM in response to jbmcnabb

See if the problem still happens in Safe mode (hold the shift key down at startup). It can take much longer to safe boot (10 min) so be patient.

How to use safe mode on your Mac - Apple Support

Safe mode forces the OS to clear certain caches and logs and do other miscellaneous housekeeping. It's non-destructive to user data so is safe to do. Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.

Does the problem persist while in Safe mode?

Exit safe mode by restarting your Mac normally and re-evaluate the issue again.

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Answer 4

jbmcnabb Author

Level 1

16 points

Nov 16, 2021 12:01 PM in response to rkaufmann87

"If it does not please verify the connections and then begin troubleshooting the connections. "

What does this mean? How does one do this?

"BTW, Another reset to try is resetting the NVRAM ( NVRAM Reset )"

I noted I tried this in my original post. No luck.

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Answer 5

rkaufmann87

Level 10

141,024 points

Nov 16, 2021 11:50 AM in response to jbmcnabb

My guess is the adapters you are using, perhaps trying a more up-to-date adapter from the Mac to the external display. Another thing to test is if you have another external display handy to test it to see if that works.

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Answer 6

jbmcnabb Author

Level 1

16 points

Nov 16, 2021 12:27 PM in response to D.I. Johnson

OK, I appreciate the step by step on troubleshooting with safe mode. Here's what I did and the results:

I booted it up with the shift key held down. I didn't get a second login window or a window that said "safe boot" like the "how to use safe mode" link said, so I booted it up w/shift key down a second time (still didn't offer those options).

Problem persisted during both safe boots and the normal startup afterwards. External monitor continues to flash "no signal"...

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Answer 7

rkaufmann87

Level 10

141,024 points

Nov 16, 2021 12:41 PM in response to jbmcnabb

"If it does not please verify the connections and then begin troubleshooting the connections. "

What does this mean? How does one do this?

What that means is you may need to try new sets of cables to see if your old cables are the problem. However, please try the Safe Mode restart and the NVRAM restart first.

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Answer 8

rkaufmann87

Level 10

141,024 points

Nov 16, 2021 12:44 PM in response to jbmcnabb

When you restart in Safe Mode you MUST MUST MUST continue holding down the Shift key on startup until it restarts in Safe Mode, as mentioned this can take up to 10 minutes. You will know it is in Safe Mode as there will be a message in the upper right of the display (normally in green) that says "Safe".

You can test your external display in Safe Mode as mentioned. Another option is to disconnect the external displays cables from the iMac, restart in Safe Mode. Then restart normally, connect the cables again and test to see if that works.

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Answer 9

jbmcnabb Author

Level 1

16 points

Nov 17, 2021 2:25 PM in response to D.I. Johnson

Good news -- I got it solved today. My external monitor is working again. I go in depth on how I eventually got it to work in my reply to rkaufmann87. Your help was much appreciated, , DI Johnson -- thank you!

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Answer 10

jbmcnabb Author

Level 1

16 points

Nov 16, 2021 11:54 AM in response to rkaufmann87

I do not have another monitor handy. This setup worked fine with the cables I had until I updated the software. So there must be a software solution...?

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Answer 11

rkaufmann87

Level 10

141,024 points

Nov 17, 2021 2:41 PM in response to jbmcnabb

That's great, I thought it was something to do with the adapter you were using!

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iMac not recognizing my external monitor after I updated to Catalina 10.15.7

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