External hard drive not visible on iMac (2019

I am going to guess the External Hard Disk's connection to the Mac is USB. Are cabling from the External Hard Disk directly to the back of the Mac's USB connections OR are you using some type of external USB hub?

I would also recommend the following so we don't have play 20 questions with you.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.

Have you restarted the computer? I have found that Sequoia is not as robust at staying up and running for as long as older macOS versions. I remember once with my Tiger OS Mac I left it running for an entire year without a single restart. I have issues with Sequoia after about a week that then clear up after a restart. This morning I was writing a reply to a question on this support forum in my browser and my Finder crashed and restarted in the background.

Which operating system version?

Do you see them neither on the desktop, nor a Finder window sidebar, nor in Disk Utility?

Have you tried restarting your computer?

What format drives?

Have you tried connecting one through a different port?