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Model A1419 / EMC 3070 / Mid 2017 / 3.4, 3.5 or 3.8 GHz Core i5 or 4.2 GHz Core i7 Kaby Lake Processor (ID iMac18,3) / Retina 5K display. Refer to the older iMac Intel 27" Retina 5K Display (Late 2014 & 2015) guides as the system is very similar.

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Replace the failing, internal SSD Blade Fusion Drive - Failing!


Fusion Drive is marked FAILING, per S.M.A.R.T. . I tried replacing internal HHD with SSD - but, wouldn’t power on. (This caused a cascade of issues - detailed below.) Returned HHD, now currently working - but, I need a permanent solution!


This is a very long story, so I’ve tried to truncate it with bullet points…

  • Everything has been working perfectly for 3+ years, other than slow restarts, slow Time Machine, slow cloning, etc.
  • Went to install a new OS. Failed, due to “S.M.A.R.T. status of “FAILING” Fusion Drive.
  • Confirmed “FAILING Fusion Drive” via Disk Utility.
  • FYI - Disk Utility only shows “Fusion Drive” - no SSD / HHD (see picture). Is this normal?

Block Image

  • Cloned 3TB Fusion Drive with Super Duper to external Crucial 2TB SSD.
  • Disassembled iMac per iFixit instruction & YouTube videos. Replaced internal HHD with SSD with OWC kit. Reassembled.
  • iMac wouldn’t start up! (Skipping hours of troubleshooting here - SMC, PRAM, reseated Ram, multiple power cords, multiple outlets, etc.).
  • Reset iMac to the original configuration. iMac wouldn’t start up. (Skipping hours of troubleshooting here…)
  • Took iMac to Simply Mac (local Apple Preferred shop). iMac start right up (WTH?!). Ran diagnostics. No issues. Passed S.M.A.R.T. Restarted several times.
  • FYI - For 2 days, I ran a secondary iMac from the cloned SSD, via an external enclosure. This is how I know that the new cloned SSD was not the issue.
  • Took iMac home, swapped internal HHD with SSD/OWC kit. iMac doesn’t start up. (%$#@). Reset with the original config. iMac doesn’t start up (no response). (Skipping hour of troubleshooting here.)
  • Reconfig back to new SSD/OWC kit. Took iMac back to Simply Mac. iMac starts up (!) - BUT, shows no OS (black screen with circle / slash / “no” icon. Ran diagnostics in Recovery Mode. This time, only the internal SSD blade shows up. However, everything else checked out (hardware, ram, etc.)
  • Took iMac home, configured to original setup. iMac works as normal again. (Thank God! Using it for 2 days now...)


DriveX has confirmed that the internal SSD, not the HHD is what is failing.

Block Image

Block Image

Block Image

(not sure what this means. It’s from the Failing SSD.)

Current Setup:

2017 27” iMac with Fusion Drive

  • 128 MB SSB Blade (Failing)
  • 3TB HHD (cloned to replacement 2TB SSD)
  • 64 GB Ram

External Drives

  • 4TB HHD RAID (Working Files, Stored Media)
  • 6TB HHD (4TB Raid Backup / 2TB System Backup — clones)
  • 4TB Time Machine (will replace soon)
  • External Desk Fan - took keep externals cool


  • 2TB SSD - purchased to replace 3TB HHD Fusion Drive


Additional Notes:

I use my machine for Graphic & Web Design. So, significant use of Adobe Photoshop, Illustrator, InDesign - but not much audio or video. However, I do stream movies, youtube, etc. a LOT.


Simply Mac called Apple and they gave the same theory we came up with: Internal SSD blade is what is failing, not the HHD. This is what is causing the SMART FAILING. This is also what is causing the intermittent no-startup. Apple suggested I split the Fusion Drive to solve the problem. But, I don’t know what new issues this will cause, as well as I’m concerned with all of the current issues I’ve had (not starting up).


  1. How can replace the Fusion Drive with the new Crucial 2T SSD, with the least amount of drama?!
  2. Do I need to split the Fusion Drive, and if so, what issues will that cause/fix?
  3. Do I need to physically remove the internal SSD blade, so that it won’t cause an issue? Or, can I bypass it somehow?
  4. Can I / Should I replace the internal SSD blade? (It’s not too expensive, but it’s a major teardown - which may cause additional issues.)
  5. What is the actual procedure I need to take? (Link?)

My biggest fear is that I will get stuck in the same no-startup scenario again. I HAVE to have my iMac for work. But, with the SMART failure, I’m concerned with even needing to reboot! Additionally, I need to make an OS update, as being on 10.15 is keeping me from updating other items, which I need.

@danj — this is extremely helpful! Thank you. In response to your 12/14/21 Update (I'm not sure where to post this):

I'm a bit intimidated about replacing the SSD Blade. (I guess I should be!) But, I'm confident I can do it. I've worked on the internals of several macs, as well as iPhones, etc. Additionally, I wasn't impressed with our local Mac shop option - I had much more experience than them. :/

I think I'm going to go with this SSD Blade (due to price and size):

I can follow the iFixIt procedures on physically replacing the drive. (Thanks for sending!)

My main concern is this:

I had originally cloned the Fusion Drive to an SSD, and then physically replaced the HDD portion of the Fusion Drive — which caused all of the issues above (no startup). Now I realize that it’s the Blade that was failing, not the HHD — So now I need to replace the Blade (as we discussed). I’m just not sure about the software procedure and the HHD replacement.

A. Can I simply clone the “Fusion Drive” and then replace both the internal SSD Blade AND the HHD (with 2TB SSD)? And, if so, when I restart, will it look the same (just faster)?

B. Or, do I have to replace the SSD Blade, but leave the slower HHD?

C. Most Importantly, do I need to do anything else, such as “slip the fusion drive” beforehand? Anything else?

Thank you so much for your time and patience!

Update (12/27/2021)

I was able to fix my iMac with the help of @danj !

In a nutshell:

  1. Made clone backup of complete Fusion Drive to a 2TB SSD with swappable USB 3 external enclosure.
  2. Replaced SSD Blade with a new OWC 1.0TB Aura Pro X2 SSD.
  3. Replaced internal HHD with 2TB SSD, using the OWC HHD to SSD Kit.
  4. Started up from new internal 2TB SSD.
  5. Initialized and installed the newest macOS on the internal 1TB SSD Blade.
  6. Used Mac’s Migration Assistant to move over the bare minimum user files, preferences, etc. (No apps, no photos, no music or movies, etc.)
  7. Restarted with new 1TB SSD Blade. Hand-installed Apps. (See why below)
  8. I will make another backup/clone of the 1TB SSD Blade/main system for safekeeping, and then…
  9. Wipe internal 2TB SSD and use it as my working volume. (I’ll store photos, music, archived client files, etc. on a 4 TB Raid external enclosure.)
  10. I’ll also make a fresh Time Machine copy of the main system since my current copy hasn’t been reliable. Thankfully, Hard Drives are cheap!

Why not use a Clone, Time Machine, or a complete Migration Assistant for Main Drive?

I’ve been migrating profiles and apps since Mac OS 9! I wanted (mostly) a clean slate with this setup. I had one app that was dated 2005!


  • The 27” 2017 iMac is now running 5X faster!
  • No issues with startup, caused by failing Fusion Drive (SSD Blade)!

Items used:

OWC 1.0TB Aura Pro X2 SSD
(Note: the first one they sent me was USED! I had them resend a new blade.)

OWC SSD Upgrade Bundle for 2012-2019 27" iMacs, 120GB 6G SSD…
(OWC’s SSD prices are ridiculous, so I got the smaller SSD with this kit and am using it on an older 21” iMac)

Western Digital 2TB WD Blue 3D NAND Internal PC SSD

Sabrent USB 3.0 to SATA External Hard Drive Lay-Flat Docking Station for 2.5 or 3.5in HDD, SSD
(I have two of these things and they work great!)

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Oh, great and mighty @danj — I'd love to hear your wisdom on such matters!


@endura I might be tall at 6'6", I'm not that great there are others who can have that title ;-}


@endura - Happy to hear it all went well! Don't forget to accept the answer - Enjoy your now more powerful system!


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First the Fusion Drive is really two distinct drives so when you state the Fusion Drive has a S.M.A.R.T. error of the drive failing that doesn't tell us which physical drive is failing: the primary (HDD) or the cache (SSD).

Most of the time it's the HDD that fails due to its mechanical nature. The only time I see the Cache SSD fail is when someone is doing quite a lot of data churn like music or video production, Fusion Drives are not well suited for that.

There are some tools which can read directly from the physical drives the S.M.A.R.T. data so you can ID which is in trouble. I use this app to check things DriveDx take a snapshot of the drives data and post it here for us to see Adding images to an existing question

Reference: List of S.M.A.R.T. Errors for HDD and SSD

While this may sound extreme often times I can get a little more life out of the SSD by reformatting it so the blocks that have failed are quarantined by the SSD's controller and over provisioning can be used to back fill the space.

What is your workflow? As that can help in aiming you to the best setup for your needs.

Update (12/14/2021)

@endura - Well you are running your system hard! At this point your data (HDD) is OK, so at this point its just a matter of replacing the NVMe/PCIe drive. Review this guide iMac 27" 2017 Blade SSD Replacement so you know what you’re getting into, I’m guessing this maybe a bit much for you to fix.

So lets review what Apple will do they will just replace the cache SSD with the exact same unit and rebuild your Fusion Drive which is not a good solution for you. They won’t put anything else in.

If you go to an independent Mac shop they can put in a different drive! Which would be the better direction for you. I would go with the minimum of 480 GB OWC Aura Pro X2 SSD or if you can swing it I would go with the 1 TB OWC Aura Pro X2 SSD

Here we are going to setup a dual drive config the NVMe/PCIe drive will become your boot drive hosting your OS and your Apps. We want to leave 1/3 of the drive as free space so your OS and apps have room to work. If you get the larger drive you’ll use some of the space for your active Adobe Photoshop, Illustrator, InDesign, Web Apps projects. Your HDD will host your finished work and media files.

iMac Intel 27" (Mid 2017-2019) SSD Blade Upgrade Bundle Görseli


iMac Intel 27" (Mid 2017-2019) SSD Blade Upgrade Bundle


iFixit Görseli


Adding images to an existing question


Çok kolay

2 - 5 minutes

iMac 27" 2017 Blade SSD Görseli


iMac 27" 2017 Blade SSD Replacement



1 - 3 hours

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Thank you, @danj ! I've run the DriveDx app and attached the results to the original post. It confirmed the SSD failed.

To follow up with your comments:

1. I'm not seeing where an actual SMART Error is listed (other than "Failing")

2. Totally clueless about "...SSD by reformatting...".

(I understand the principle I think. It's kind of like skipping bad block from HDD optimization, right?)

But, the execution....?!

3. Not sure what you mean by workflow. If you mean, "how do I use my machine"? Heavy use of Adobe Photoshop, Illustrator, InDesign, Web Apps & YouTube.

Thanks again!!


@danj — do you have any thoughts on setting up the software side of the Fusion Drive?



1 - The reason is the new APFS file system doesn't offer out a way to get to it cleanly from within DriveDx. But it offers a go/no go in the trial version.

2 - Basically you use Disk Utility to format the discreet drive. But with a Fusion Drive you can't get to the blade SSD to format it as its setup as a cache drive. Its only when you put in a replacement drive will you have a chance to format the replacement drive. Here's the Apple help file Erase a volume using Disk Utility on Mac, How to format a drive on a Mac

3 - Yes, Workflow is your work and how you use your system to perform it.

Fusion Drives, why we don't use them any more

I don't recommend Fusion Drives for advanced Amateurs (music or video production students) or working pro's.

The data churn is too great within these workflows. A home user who writes, watches YouTube vids or produces small home music and videos its OK.

But why are you so set on Fusion Drives?? You will just burn out the replacement blade SSD overtime.

Remember a Fusion Drive SSD is small not very large so it has less room to work from and as each change causes rewrites the cache is constantly altered. While one could argue a dual discreet drive setup is no different as the boot drive would be used for VRAM as well as OS & application caching. These are not as heavy a load and as we are working with a larger drive with at least 1/4 left free the SSD's micro-controller has more space to work with for write leveling so the drive is more evenly worn.

SSD costs have dropped so significantly there is no reason not to just go with a SSD as your boot drive and leverage your HDD as a data drive.

Apple made a fateful flaw not thinking how to merge SSD's and HDD's in a more productive way. Fusion Drives where a great start when SSD's where costly. But a better design would have been file type marking and treating multiple disks as a flat space similar to the Just a Bunch of Disks (JBoD) is layed-out but unlike them the OS places the file magically to a set of smart rules so OS files are on the SSD, yet data files are placed on the HDD (this is a bit of an over simplistic example).

Apples current direction using more tightly bounded RAM and Flash is still faster for sure! But at the cost of not being expandable or serviceable (full logic board replacement needed)!


@danj thanks again! Great point with nixing the Fusion Drive! I'm totally cool with it - I'm just trying to preserve my setup, without having to reinstall and re-setup everything (OS, Apps, Setting, thousands of Fonts, etc).

So, just to be clear, you are saying:

1. Replace the failing SSD Blade, and close up the machine.

2. Install an OS on the SSD Blade (via Wifi). The iMac should now recognize the two separate drives as just that: two separate drives. No need to reemploy the Fusion Drive! (The "Fusion" should automatically break when I switch the SSD Blades. No Terminal needed.)

3. Clone back over my files to the internal HHD (?)

A. Is that correct?

B. Is there any benefit to replacing the internal HHD with the new SSD I bought? (If so, it would save me a step, since the "Fusion Drive" was already cloned to it!)

C. Any idea why the machine wouldn't boot when took out the HHD and replace with the SSD? You would think that it would just say "no system" - instead of acting totally dead for a while!



1 - Yes, just replace the blade SSD, I would hold off buttoning the system fully, I would use painters tape to hold the display on until your sure everything is good.

2 - Installing the OS doesn't need a network connection, if you've already have the OS installer on your system (HDD in this case) It's much faster running directly on the system. I tend to use USB thumb drives setup as OS installers. Apple keeps altering things so dependent on what macOS you have currently you might need to delete the Fusion Drive setup (that won't disrupt what's on the HD)

3 - NO!!! We don't clone! Do you still use an old crank phone?? Or even a corded dial phone - So passé! We use Apples Migration Assistant for the task How to transfer data to your new Mac from your old Mac Here we are moving the user accounts and apps to the SSD (not your data and media files) selectively moving things over.

To your other points

A - As I've stated here

B - Sure if the SSD is large enough but remember it's only SATA III (6.0 Gb/s) whereas the NVMe PCIe x4 is 8.0 Gb/s with less overhead! At which point you'll also need to get an external case to hold your 3.5" HDD to migrate your data back to the new 2.5" SSD and you'll loose to dual connection of the two ports which lowers the collective throughput.

C - The SSD cache drive was getting in the way and as its not a bootable drive it wound't have offered a boot up and yet it is present so the CPU was Flummox! Not knowing what to do IT could see the drive yet it couldn't access its boot sectors to even know if it had an OS present or not. This is one of the half in and have out conditions a computer can get into.

I would stick with the plan getting a 512GB or 1 TB blade SSD either Apple or OWC to replace your bad Fusion Drive caching blade SSD. Leave your HDD alone, migrate your Apps and user accounts to the SSD and fi you have a larger project use the SSD to work off of. Make sure to leave 1/4 to 1/3 unused space on the SSD. The more the better!


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