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A2115 / 2019 / Processors from 3.0 GHz 6-core i5, up to 3.6 GHz 8-core i9. Released March 19, 2019.

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What are my options to replace a dead Fusion Drive?

Hello everyone, I currently have a 2019 iMac 27" with a Fusion Drive. One day it quit on me and now I cannot boot at all, and only see a "circle with slash" error. I've gone into recovery and Disk Utility does not properly detect my HDD from what it looks like. I've also tried to reconnect the fusion drives but it provides an error saying that I only have one drive. After some research, I have come across the below solutions,

  1. Replace the internal HDD with a large SSD. This is my preferred solution, but I would also like to disable the SSD cache drive. I believe having just the one SSD will be more than enough for me. Is it possible to disable the SSD cache without physically removing it?
  2. Booting from an external drive. Second preferred method. I don't mind having an external connected. Would a USB3+SSD combo suffice for this or should I be looking at Thunderbolt+NVMe? Also, would the (possibly) 2 failing internal drives have any affects on my system?
  3. Replacing the HDD and SSD. If I have to I can go this route. Though if I did this, could I JUST replace the SSD blade with a larger capacity one, and have that do all of the work? It would at least save me some effort and cost to buying 2 drives and also various adapter kits, etc

Any help is greatly appreciated! Please let me know if there's any more information I can provide. I'm currently toying with recovery mode, and am going to try installing macOS onto an extra external SSD that I have, just to see how it goes or if it's even possible

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1 - Sadly, you will need to pull the blade SSD as it gets in the way trying to sync up withe the new drive which you don’t want to do here.

2 - Think how you use you car, just motoring around town with the kids or driving great distances on the Autobahn. Very different use cases! For most the internal drive is the better as USB is on the slow side Vs SATA and PCI is the fastest internal, with Thunderbolt3 with a RAIDed drive set the fastest.

Not sure I follow you here with two failed drives. A Fusion Drive set is seen as one drive so if you physically loose one drive the second is useless. That is what dual independent drives are better and using the better technology for the function here using a SSD for the boot drive and Applications and free space for virtual ram, caching and scratch space if the app uses it. Then use the HHD as your data drive holding what’s important. Of coarse backing up as well to an external drive as well to be fully safe.

3 - Sure getting a bigger blade SSD also works. But depending on your workflow it may not be the best option!

Let’s look at this in a physical model… You have a chalkboard and your have a single sheet of paper how many times can you write on the chalkboard wiping off once you’ve filled the board? Now how many times can you erase the paper after filling it?

Clearly the chalkboard is infinite! The sheet of paper only has so many times before it’s either torn or just not readable! This is the difference between a HDD and a SSD a HDD can be written to many more times than a SSD, barring a physical drive failure.

Conclusion: What is your usage of the system will make a big difference on what it the best option for you. Just like the given car you drive :eats your usage need.

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Thank you for the reply Dan

1. How about if I was willing to keep the blade, can I install a SSD in place of the HDD and utilize both of them in a "Fusion Drive" configuration?

2. Sorry, what I meant by the 2 failing drives was that I wasn't sure which drive was actually failing, the HDD or the SSD blade. I wanted to make sure that IF I did go the external route, that leaving those 2 drives in the computer wouldn't affect my computer's performance or cause other issues down the road.

3. I do know about the life of an SSD, and definitely not worried about that in my case.

A bit more since I was over the limit....sorry!

Additional info: My needs are pretty low. This is used in a schools administration office (dont ask why our IT can't repair it for us..) and most of my work is actually done through Google Drive. I am not worried about amazing performance or longevity to be honest.

So having said that, I had made some strides:

- I was able to test out an external SSD+usb3 with macOS installed and will say it runs pretty decently

- I was then able to use Disk Utility to format out the SSD drive (128gb), then booted into recovery again and installed macOS directly on this drive. I then found that I'm able to boot into the imac with no issues with just this drive. Obviously it's a smaller size but I don't honestly think I need more, and I may just use an external hard drive for file storage as needed.

Lastl one....

A new question however, is that I am getting a warning message upon boot that there is an unusable drive that can't be used. I assume it's referring to the HDD which is just kaput and unusable. Is there a way to disable it? Is it doing me any harm just sitting there? All I did to "unlink" the Fusion Drive was format the internal SSD blade, so not sure if there's any terminal commands or something that may need to be done.

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1 - There is no benefit setting up a dual SSD Fusion Dive.

2 - it really doesn’t matter which failed if you go with an external drive.the performance with be lower based on which connection and drive setup you go with externally.

3 - don’t go to small! My rule of thumb is to end up with 1/4 to 1/3 of the drive is not used for the needed elbow room.

The 128 Cache drive is really to small to be really useful and given its use it’s life span is limited. Best to replace the internal drives.

Until you remove the HDD it will continue to report the bad drive.

Truly, it’s time to get off the fence. Follow the guides pull the two drives and decide which setup makes sense a single SATA SSD or PCI SSD. Or go with the more powerful dual drive setup.

In either case pull both drives unless you really want to do this again in a years time if not sooner.

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Don’t forget to score the answer and accept it, if we have answered your concerns and questions.

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@danj Hi Dan, still experimenting here. For the moment I have chosen to erase the internal blade and install macOS onto it, and connect an external SSD to hold the majority of my downloads. This will honestly be more than enough for me. However I'm currently having an issue where whenever the computer goes into sleep for maybe more than a minute, there is a kernel panic and it resets. Any advice for this? Can't post much of it due to the character limit.

panic(cpu 0 caller 0xffffff800bef928a): AppleAHCIDiskQueueManager::setPowerState(0xffffff90628a3940 : 0xffffff800ddae098, 3 -> 0) timed out after 102174 ms @IOServicePM.cpp:5547

Panicked task 0xffffff99fb28f218: 216 threads: pid 0: kernel_task

Backtrace (CPU 0), panicked thread: 0xffffff99fa281b30, Frame : Return Address

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@Andrew Hong - who’s SSD did you install this is a disk error condition.

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just found this post after trying to work out why my iMac2017 starting having issues a couple of days after replacing the mechanical HDD in my mac, which was fusion..... is the issue related, in changing the mechanical HDD for a SSD? Just don't see how/why

panic(cpu 0 caller

0xffffff800c0231bc): Bad tailq elm 0xffffff86cd6cff30 prev->next != elm

@3131 @vfs_cache.c:3131

Panicked task

0xffffff9063bd4258: 6 threads: pid 1: launchd

Backtrace (CPU 0),

panicked thread: 0xffffff90620cb598, Frame : Return Address

0xfffffffc00b67820 : 0xffffff800be70c7d

(repeated a fair few

times!)

Process name

corresponding to current thread (0xffffff90620cb598): launchd

Mac OS version:

22G436

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You do realize there is a hidden SSD on the backside of the logic board iMac Intel 27" Retina 5K Display 2019 Blade SSD Replacement this SSD is a cache drive to your SATA HD which combined is the Fusion Drive set.


You do need to take it out with the newer macOS’s installed as the systems firmware is now used holding a pointer to this drive for caching, the error you are getting is the missing of the expected HD mate is gone! Where O where has it gone? To the dust bin I suspect which is where this SSD likely needs to go as well.


I’ve managed to salvaged a few using a windows system to force an exFAT format to use as an external drive or to then put back in (if it’s the larger 256 GB drive) making it the boot drive. Likely you have the smaller 128 GB drive which is not very useful on its own.

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Andrew Hong zal eeuwig dankbaar zijn.
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