iMac no longer boots after security update- Does not see internal SSD

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iMac Intel 21.5" Retina 4K Display (2017)

A1418 / EMC 3069 / 2017 / Processeur quadricoeur i5 à 3,0 GHz, quadricoeur i5 à 3,4 GHz ou quadricoeur i7 de 3,6GHz. Sorti le 8 Juin 2017.

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Saltshaker185 @saltshaker185

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Publié: 14 sept. 2025

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iMac no longer boots after security update- Does not see internal SSD

MacOS chose to automatically update my system last night, and I woke up to a computer which wouldn't boot up. The system instantly shows the apple logo but will do nothing else. NVRAM reset works right away, but recovery mode (online) and booting from a USB key shortcuts need to be held for about 15 min before they're recognized and work.

When in disk utility on either option, it does not seem to see my SSD anymore. It shows a 2gb partition (recovery partition I believe), and then a 30gb partition which the first aid fails. It has a fairly new 500gb samsung SSD installed which has been working great before the update.

Any ideas before I open up this machine to try and test the drive? Extremely frustrating for an update to cause this. Thanks!

This was a fusion drive before I purchased it. The Fusion drive was swapped out for the SSD. Only other upgrade was a ram upgrade at the same time.

The computer is a Mid 2017 21.5" Imac. Thanks!

Update:
Removed the hidden original SSD, but still having the same issues. Only drive I can see is this one, which doesn't help me.

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Can you tell us the model as Apple has produced quite a very versions of this system.

Also was this a Fusion Drive model when you bought it? That important to know.

15 sept. 2025 par DanJ

@danj See above, meant to reply to you.

15 sept. 2025 par Saltshaker185

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Solution retenue

DanJ @danj

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@saltshaker185 - Ah! We had a Fusion Drive setup! Did you remove the second hidden drive? Review this guide to pull it Changement SSD blade iMac Intel 21,5" Retina 4K (2017) it gets in the way when you only replace the SATA HDD for a SSD drive. Which is likely your issue here.

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Might be on to something here. I'll split her open and report back. Thanks Dan!

15 sept. 2025 par Saltshaker185

@danj old ssd removed, but still having issues viewing the new one. Ideas on next steps? Thanks!

15 sept. 2025 par Saltshaker185

@saltshaker185 - Almost there! The new SSD still has pointers to the second SSD (was the HDD) when you replaced it the blade SSD which was the caching drive to the HDD still was alive and just assumed the new drive needed to be cached now that it also failed (often does) the cache link is now severed again! It's the two drives together which make the Fusion Drive Set.

Sadly you need to fully erase, re-partition and re-install the OS. I find it's best to create a bootable USB thumb drive OS installer so you can make sure you fully nuke the drive as there are hidden partitions which are getting in the way. What's important here is we can't have running from the SSD. Sadly any data on it presently will be lost. You might try using First Aid from the Thumb drive and if the data is important you might find Drive Genius a try.

16 sept. 2025 par DanJ

@danj So the computer no longer has a blade drive in it, only the SSD which replaced the HDD. I'm still unable to see the SSD in disk utility and as such, i'm not sure how to erase and repartition it. Any idea on how to get it to show up? I have a bootable USB with Monterey on it, I feel like I saw some comment about Monterery maybe not being the best version for this? Let me know your thoughts. Thanks!

16 sept. 2025 par Saltshaker185

@saltshaker185 - Yea, this is confusing! Which is why I explained a bit how a Fusion Drive is created. What you are seeing is not complete! As there are other hidden partitions and links that are created between the two drives they are still present which is now messing you up.

To erase the drive you need a bootable external drive. For now download on a different Mac the version of macOS the system is using now from the Apple Store then copy it onto a clean formatted USB thumb drive (32GB) using Disk Utility in the Utility's Folder or menu access as GUID Journaled File System. Then follow this guide to convert the installer file into a bootable version Create a bootable installer for macOS.

Now with this plugged into your struggling iMac reboot it should now be running off of this USB OS, here again we reformat the 2TB Samsung SSD. And partition it with one visible partition. The OS installer will create some hidden partitions when it is run.

That should do It!

16 sept. 2025 par DanJ

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Can you tell us the model as Apple has produced quite a very versions of this system.

Also was this a Fusion Drive model when you bought it? That important to know. — DanJ
Might be on to something here. I'll split her open and report back. Thanks Dan! — Saltshaker185
@danj old ssd removed, but still having issues viewing the new one. Ideas on next steps? Thanks! — Saltshaker185
@saltshaker185 - Almost there! The new SSD still has pointers to the second SSD (was the HDD) when you replaced it the blade SSD which was the caching drive to the HDD still was alive and just assumed the new drive needed to be cached now that it also failed (often does) the cache link is now severed again! It's the two drives together which make the Fusion Drive Set.

Sadly you need to fully erase, re-partition and re-install the OS. I find it's best to create a bootable USB thumb drive OS installer so you can make sure you fully nuke the drive as there are hidden partitions which are getting in the way. What's important here is we can't have running from the SSD. Sadly any data on it presently will be lost. You might try using First Aid from the Thumb drive and if the data is important you might find Drive Genius a try. — DanJ
@danj So the computer no longer has a blade drive in it, only the SSD which replaced the HDD. I'm still unable to see the SSD in disk utility and as such, i'm not sure how to erase and repartition it. Any idea on how to get it to show up? I have a bootable USB with Monterey on it, I feel like I saw some comment about Monterery maybe not being the best version for this? Let me know your thoughts. Thanks! — Saltshaker185
@saltshaker185 - Yea, this is confusing! Which is why I explained a bit how a Fusion Drive is created. What you are seeing is not complete! As there are other hidden partitions and links that are created between the two drives they are still present which is now messing you up.

To erase the drive you need a bootable external drive. For now download on a different Mac the version of macOS the system is using now from the Apple Store then copy it onto a clean formatted USB thumb drive (32GB) using Disk Utility in the Utility's Folder or menu access as GUID Journaled File System. Then follow this guide to convert the installer file into a bootable version Create a bootable installer for macOS.

Now with this plugged into your struggling iMac reboot it should now be running off of this USB OS, here again we reformat the 2TB Samsung SSD. And partition it with one visible partition. The OS installer will create some hidden partitions when it is run.

That should do It! — DanJ

Rashmi Kandhil @rashmikand95190 Rep: 1 3 · Answer 1 · 2025-09-15T05:04:37-07:00

Rashmi Kandhil @rashmikand95190

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Publié: 15 sept. 2025

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It sounds like the macOS update may have corrupted your SSD’s file system or failed to detect the drive’s firmware, so testing the SSD externally with another machine or enclosure would be the next best step before reinstalling macOS.

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Thanks Rashmi, I was afraid i'd be splitting it open to do that. Do you have a recommended software or process you use for this? (Free if possible) It's been a few years since i've diagnosed drives, so figure there may be something more streamlined these days.

15 sept. 2025 par Saltshaker185

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iMac Intel 21.5" Retina 4K Display (2017)

iMac no longer boots after security update- Does not see internal SSD

Update (09/15/25)

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