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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. Benutze die Anleitungen des iMac Intel 27" Retina 5K Display (Ende 2014 & 2015), da das Gerät sehr ähnlich ist.

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Fusion Drive Replacement Problems

Here’s my situation! In Oct 2019 I replaced the original Fusion drive with 1TB SSD drive but I can’t remember if I split up the Fusion drive before I installed the new SSD. I’ve installed MacOS on that drive and it’s been running except for the drive access speeds being very slow!

So I thought I’d replace the existing 128GB PCIe SSD (that came with the Fusion Drive) with a faster 1TB version. I just got that done, but now Disk Utility doesn’t see the new PCIe SSD - it does see the old SSD that I replaced the Fusion drive with.

That got me thinking and I can’t for the life of me remember if I ever split the Fusion drive up before I did the original SSD replacement in 2019. Also, I don’t think I ever saw the 128GB PCIe drive ever show up in Disk Utility before the replacement a couple of days ago.

Do you think I forgot to split it up which is why I can’t see that drive (the original 128GB SSD or my new 1TB SSD)? I’ve reset the PRAM and SMC and rebooted into recovery etc.

I don’t want to have to open up the iMac again, so if there’s anything you guys can think of to try please let me know! (diskutil list only shows the SATA SSD drive)



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The Fusion drives SSD is not visible within the finder or About this Mac storage section as its a cache drive which is hidden!

Depending on what your macOS is and how you are using Disk Utility it may not be visible either!

We also need to describe the drives better here a Fusion Drive is the two drives in combo. Each drive to them selves are what they are! So you had a HDD and a impish SSD for your Fusion drive set.

You took the SATA HDD out and put in a SSD in its place, So you ended up with a double SSD Fusion drive which explains your slow drive problem as you have two SSD’s fighting it out!

So you do need to break the Fusion set but different macOS’s do things differently! So what do you have?

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Thanks for the reply. You’re exactly right. I’m running Big Sur (10.14)


Here's what I would follow: Split Your Fusion Drive Apart

You probably want to disconnect your SATA drive and install a fresh copy of the OS on your blade SSD, then reconnect your SATA SSD.

One last point don't go cheap! Stick with either a real Apple blade SSD or go with the OWC Aura SSD. I've pulled out so many M.2 SSD's in every Mac model. While day one these SSD will work after you've had enough data churn or get full the drive will crap out!

It's like someone throwing saw dust into the cars crankcase or transmission to make the car stop knocking or grinding so you are none the wiser when you buy it.


I checked out that article on splitting your Fusion drive. When I enter diskutil cs list in terminal I get "No CoreStorage logical volume groups found"

Do you think I need to put my old blade SSD back along with the original Fusion SATA drive?


@stevodevo - I've always split before swapping the drives so thats what I would do.


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Stevo Devo wird auf ewig dankbar sein.

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