2019 27” Mac won’t install any OS

Question

2019 27” Mac won’t install any OS

Hi all, I’ve had to erase/reset my 27” Mac because it wouldn’t log in and wouldn’t boot from a back up. But now that the Fusion Drive is erased I can’t install any OS at all.

I’ve tried recovery and Internet recovery mode.

I’ve rest NVRAM and SMC.

I've erased and reformatted the Fusion Drive. I even reset it using terminal.

I’ve even tried installing OS on an external drive. It always stops installing and come up with errors.

No OS will install… please help

iMac 27″

Posted on Aug 16, 2025 12:34 AM

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Answer 1

Old Toad

Level 10

214,368 points

Aug 16, 2025 11:25 AM in response to mattfromgarbutt

It might be time to make an appointment at the nearest Apple Genius Bar or Apple Authorized Service Provider for a free diagnostic check (The AASP will charge for the diagnosis but wave it if you have them repair it).

They can refuse the Fusion drive if it's been un-fused, reinstall the latest system the iMac can run.

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Answer 2

Old Toad

Level 10

214,368 points

Aug 23, 2025 10:42 AM in response to mattfromgarbutt

Here's some food for thought: a 10 Core Mac Mini M4 with 16 GB RAM and 1 TB SSD ($999 + tax) with a 27" LG 4k monitor ($250 + tax from Amazon.com) is $1249 + tax which is $560 less than a similarly configured new 24" iMac ($1799 + tax) re 07/2025.

I got the same Mini but with a 32" LG monitor w/speakers for only $60 more.

The bottom line: a Mini M4 + 3rd party monitor is much more economical than a new M4 iMac.

As I said just some food for thought.

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Answer 3

WheelieNick

Level 8

45,647 points

Aug 16, 2025 01:12 AM in response to mattfromgarbutt

What is the exact error that you;re facing? That way we can help you resolve the issue better

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Answer 4

rkaufmann87

Level 10

148,104 points

Aug 16, 2025 01:32 AM in response to mattfromgarbutt

Can you boot into the recovery partition and install Mac OS from there. Also have you checked to see the Fusion Drive is still fused?

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Answer 5

Aug 16, 2025 01:35 AM in response to WheelieNick

It’s either:

the installation could not be completed, or

The file couldn’t be opened because it isn’t in the correct format (“The file might be corrupted, truncated or in an unexpected format”)

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Answer 6

Aug 16, 2025 01:36 AM in response to rkaufmann87

how do I do that please?

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Answer 7

WheelieNick

Level 8

45,647 points

Aug 16, 2025 03:55 AM in response to mattfromgarbutt

Boot the mac with cmd+R and select reinstall macOS

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Answer 8

Aug 16, 2025 04:29 AM in response to WheelieNick

You clearly didn’t read my post, but thanks anyway

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Answer 9

Aug 22, 2025 11:59 PM in response to mattfromgarbutt

Hi everyone, I took it to the Genius Bar and they ran a full diagnostic. Logic board failed to respond. Well that’s that. RIP iMac. Thanks for advice anyway guys!

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