Can I install earlier macOS after deleting macOS Sequoia on iMac 24"?

If you purchased this machine in 2018, it probably was a 21.5" iMac released in 2017. Apple stopped making 24" iMacs in Late 2009 when the first 21.5" iMac came out – and did not resume making 24" iMacs until 2021, when the 24" M1 iMac came out.

There were two 21.5" 2017 iMacs: a Retina model, and a non-Retina model.

For both, the original OS (and lowest version of macOS that they can run) was macOS 10.12.4 or 10.12.5 (Sierra). The most recent version of macOS that either can run is Ventura.

I'm not sure why you would want to go back to Catalina, given that

• Catalina won't run 32-bit applications – the way that Sierra, High Sierra, or Mojave would; and
• Catalina isn't one of the "most recent three" – the way that Ventura is.

If your iMac is indeed from 2017, it would seem that Catalina is one of the worst choices you could make in terms of application compatibility!

Also, I do not see how you could be running Sequoia on that iMac unless you used some sort of hack to install it. To be running Sequoia on a 21.5" iMac in a supported way, that iMac would need to be from 2019. In which case the earliest version of macOS that runs on it would be macOS 10.14.4 (Mojave) – and once again, Catalina would seem to be a bad choice in terms of application compatibility.

charlesfromsetauket wrote:

I am open to any suggestion as to which OS to install. This computer would be used primarily for emails, documents and iTunes library.

At this point, I would suggest

• Staying on Sequoia
• Getting an external SSD, cloning your Sequoia system to it, and making it your startup disk
• Giving the system a little while to settle down, just in case Spotlight decides to re-index everything

Then evaluate the performance. If it has improved sufficiently, you probably will want to stay on Sequoia. If not, you could boot off the Sequoia installation on the internal drive, erase the external SSD, and do a clean install of some lower version of macOS on it.

Note that

• Sonoma and Ventura are the only others among the "most recent three" that get the most love from vendors like Microsoft and Adobe
• Ventura is almost certain to fall off that list some time this September, October, or November
• With any macOS downgrade, it's possible that you would run into the problem that the versions of Music, TV, and Photos on the lower version of macOS would NOT understand libraries used with newer versions (due to the new versions having performed a one-way upgrade).
• Even with an earlier version of the OS, if you have a mechanical hard drive or Fusion Drive now, you're likely to need an external SSD to improve performance … so might as well see if it makes Sequoia run smoothly before concluding that you need BOTH an external SSD and a macOS downgrade.

Reinstall your computer’s original version of macOS from the internet: Press and hold Shift-Option-Command-R until a spinning globe appears, then release the keys.

See: Reinstall macOS - Apple Support

What type of drive does that iMac have?

Those iMacs came with 1 TB mechanical hard drives, 1 TB Fusion Drives, or flash storage (SSDs). The first two were both slow. If you have either, getting an external SSD, cloning your current system onto it, and using that external SSD as your startup disk might make performance better. Especially with regards to the time it takes to start up the Mac, and to launch applications.

That Mac supports

• USB-A (USB 3.0)
• USB-C (USB 3.1 Gen 2)
• Thunderbolt 3

connections to external drives.

So you shouldn't have any trouble finding a compatible external SSD.

charlesfromsetauket wrote:

The iMAC was from 2018 and the original OS was Catalina. Sorry for omitting that info from original posting.

The post is somewhat confusing. There was no 2018 iMac and the only iMac that shipped with Catalina by default was the 2020 27" Retina 5K.

You cannot install an OS that predates the one the computer shipped with.

A 24-inch iMac cannot Run macOS Catalina.