I keep getting "Installation of macOS cannot continue. ...download important data etc." warning

Predicated on this computer Qualifies to run macOS 15 Sequoia

The below link should answer this question very quickly.

macOS Sequoia compatible Computers 

If Yes you may consider this User Tip from fellow contributor @ Jack- 19

link >> Use an external SSD as your startup disk … - Apple Community

During the Installation process

" The installer needs a stable internet connection to get firmware and other information specific to your Mac. Make sure that your Mac is connected to the internet. "

Johnsm8 wrote:

Perhaps the OP (Eric) will return to advise us of progress.

I do hope so.

If the iMac in question is a 2017 model, which can only update as far as Ventura, if an external disk is used to start the computer, can you confirm that Sequoia cannot be loaded onto the external drive to run the iMac? I have a notion that the external drive must also be restricted to Ventura.

Thanks.

Yes the Age of the computer will determine this, regardless of Internal or External Drive

Hello!

You mention that you are on macOS 12.6. Have you tried updating you main computer?

Before so doing, you might like to carry out this procedure.

1. Start up from macOS Recovery.
2. When you see the utilities window in Recovery, choose Utilities > Terminal from the menu bar.
3. Type diskutil resetFusion in the Terminal window, then press Return.
4. Type Yes (with a capital Y) when prompted, then press Return.
5. When Terminal indicates that the operation was successful, quit Terminal to return to the utilities window.
6. Choose Reinstall macOS, then follow the onscreen instructions to reinstall the Mac operating system. Your Mac will restart from your Fusion Drive when finished.

Reference:-

How to fix a split Fusion Drive – Apple Support (UK)

If the iMac in question is a 2017 model, which can only update as far as Ventura, if an external disk is used to start the computer, can you confirm that Sequoia cannot be loaded onto the external drive to run the iMac? I have a notion that the external drive must also be restricted to Ventura.

I've often wondered what part/section of a Mac is so redundant that it prevents moving on to the next macOS.

Do you have any idea?

Yes the Age of the computer will determine this, regardless of Internal or External Drive

Thank you for your confirmation. 🙂

If Yes you may consider this User Tip from fellow contributor 'Jack'

Use an external SSD as your startup disk … - Apple Community

I concur. Jack's method worked well for me! 😄

Good suggestion, PRP_53!

When in Disk Utility, you can use View menu to show "All Devices" to ensure that you see each component meaning the SSD Drive and the 3 TB Spinner / Mechanical Drive

Please will you show/explain HOW that may be achieved?

Thanks

(Using Terminal, this command should produce the answer required: diskutil list physical )

edsass wrote:

Thank you for your suggestion to reset the Fusion drive.

You are most welcome, Eric. I'm just an ordinary end-user, not one of the gurus who post here! 😄

Unfortunately, when I tried it I got a "command not found" reply in Terminal. I am not sure which of the available commands to use as a viable option. I know that it is a Fusion drive that should have 3TB of storage but as I said that machine was compromised during a reboot, I have no idea what went wrong but I watched it freak out and shutdown. When I rebooted it next it came on with only 121GB of storage available. Every time I turn it on I also get a pop-up asking me what to do with an uninitialized drive and out of fear just click "Ignore."

It is a back-up machine now but I was hoping to make it more useful. Sorry if htat's too much information. It's just been a frustration for me.

Any other help you might be able to give will be greatly appreciated.

-Eric

I believe that the SSD in your computer should be APPLE SSD SM0128L - 121.33 GB 

I'd like to suggest that you carry out the procedure again. Copy and paste the instruction into Terminal so that no mistake is made - diskutil resetFusion

Please advise of your result.

Thanks.

Sorry

When in Disk Utility, you can use View menu to show "All Devices" to ensure that you see each component meaning the SSD Drive and the 3 TB Spinner / Mechanical Drive

There are no iMac, I can find, that qualify to run macOS 15 Sequoia and that come configured with a 3TB and 128 GB SSD drive making the configuration a Fusion Drive

There is a iMac (Retina 5K, 27-inch, 2019)  which if Custom Ordered

• 3.7GHz
  • 2TB Fusion Drive
  • Configurable to 3TB Fusion Drive or 512GB, 1TB, or 2TB SSD

Perhaps if you can provide a Screen Shot from About this Mac >> without the serial number appearing

We maybe about to better identify exact what machine is having this issue

Thank you PRP_53 🙂 I appreciate you showing me a screenshot.

Sadly, though, I do not believe that the sizes of the two parts of a Fusion Drive will show up separately in Disk Utility.

Perhaps someone else will check and advise us?

Respectfully

Believe as in this iMac computer qualifies to run Sequoia and has " I believe that the SSD in your computer should be APPLE SSD SM0128L - 121.33 GB  " ??

I do not have a Fusion Drive to Test this process

Though, based upon a very reputable posting by another Contributor

Disk Utilities should work

The " proof will be in the pudding " lol

Perhaps the OP (Eric) will return to advise us of progress.

I do hope so.

If the iMac in question is a 2017 model, which can only update as far as Ventura, if an external disk is used to start the computer, can you confirm that Sequoia cannot be loaded onto the external drive to run the iMac? I have a notion that the external drive must also be restricted to Ventura.

Thanks.

Great minds think alike ? 👍

Thank you for the compliment

Not often seen here but should be ?

👍