No way to reinstall MacOS on old iMac

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rk2008 Author

Level 1

0 points

No way to reinstall MacOS on old iMac

My dad handed down his old iMac (2015?) to us, for our kids to learn and do their homework on.

Without thinking twice, I dutifully erased the hard drive and began as MacOS re-install.

What I didn't expect was an error, apparently because the certificate of the old OS had expired.

My next step was to create a boot disk on a USB drive. So I downloaded Sequuoia (15.0.1) and attempted to boot from that, but the old iMac doesn't support that OS. I then tried to create a boot disk for an older OS like El Capitan, but my newish MacBook Pro won't allow me to run the Install.pkg of the older OS to create a boot disk.

So I'm now stuck with a bricked iMac. The recovery disk can't install the OS, and I can't create an external boot disk that either the iMac or my own Mac supports.

Any ideas on what to try next are appreciated!

Earlier Mac models

Posted on Oct 18, 2024 5:11 AM

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Answer 1

Oct 18, 2024 6:13 AM in response to rk2008

Exactly which MacBook Pro do you have?

You can find that info by clicking Apple menu  > About This Mac.

If we know the model and year perhaps we can help you find an OS version that both the old and newer Mac can run.

If you can reboot the old iMac (2015?) in Recovery mode using Option-Command-R you may be offered the ability to reinstall the latest OS that it is compatible with. This will require an internet connection, and a wired ethernet cable direct to the router will be better than wi-fi for this.

How to reinstall macOS - Apple Support

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Answer 2

Oct 18, 2024 8:03 AM in response to rk2008

You wrote: ❝ iMac - I haven't gotten far enough to find out the spec. ❞

You might find the identity of the iMac if you input the serial number into Apple's check coverage lookup tool here:

Check Your Service and Support Coverage - Apple Support

You can find the iMac serial number on a label on the underside of the pedestal.

I believe your 2020 MBP can run macOS 10.15 Catalina and therefor should be able to create a bootable Catalina USB drive. If the iMac is a mid-2015 thru 2019 model, then it also would be capable of running macOS Catalina.

It's fairly easy to create a bootable USB with a Catalina installer. The guidance for that is here:

Create a bootable installer for macOS - Apple Support

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Answer 3

HWTech

Level 9

59,984 points

Oct 18, 2024 8:18 AM in response to rk2008

What is the exact model of your iMac? You can get this information by entering the system serial number on the check coverage page here (please don't post the serial number on the forums since it is considered personal information):

Check Your Service and Support Coverage - Apple Support

Edit: If you do have an actual 2015 iMac, then it supports Big Sur which a 2020 laptop also supports.

Here is an Apple article with instructions & links for various macOS installers:

How to download and install macOS - Apple Support

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Answer 4

rk2008 Author

Level 1

0 points

Oct 18, 2024 7:26 AM in response to D.I. Johnson

MacBook Pro is a 13-inch 2020 (2GHz Quad-Core Intel Core i5) running Sequoia 15.0.1

iMac - I haven't gotten far enough to find out the spec

I tried creating an El Capitan boot disk on my MacBook via both the GUI and the Terminal, but get:

installer: Error - This version of OS X 10.11 cannot be installed on this computer.

My fiddling has made things worse as well - the iMac now doesn't have a recovery disk, so I can only boot via the internet (which attempts and fails to install Lion) or via a boot disk (neither Catalan nor Sequoia will install on the machine - I get the circle with a line through it symbol).

I can only create boot disks with OS versions in the App Store (older versions give the above error) and the newer OS versions in the App Store don't seem to be supported by the iMac.

I even tried to extract out the .pkg download of El Capitan on the iMac in recover mode, but it fails saying that I need to install a Mac OS first.

Going round in circles, and all because Apple let their signing certificates lapse in 2018/2019 😅

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Answer 5

den.thed

Level 10

81,640 points

Oct 18, 2024 7:42 AM in response to rk2008

If making an installer doesn't work, have your Apple Store or AASP reinstall the MacOS.

see > Find Locations

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