Fusion and hard drive replacement on late 2012 imac

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Level 2

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Fusion and hard drive replacement on late 2012 imac

Hi,

I’ve got a late-2012 27” iMac, and the internal hard drive is failing. I’m planning to replace it but would appreciate advice on something.

The original hard drive in this computer is a “fusion” drive. I’m under the impression that this means it has both a larger mechanical drive plus a smaller solid state drive, but I read conflicting answers as to whether both devices are in a single 3.5” bay or if the mechanical drive is in the 3.5” bay and the solid state storage is actually a chip buried elsewhere in the computer. (The replacement instructions for the drive itself sound tough but manageable, but the instructions to remove the chip sound prohibitively difficult.)

My questions are:

Any way for me to tell which configuration I have?
If do in fact have the “separate chip” configuration, what would happen if I replace the mechanical drive in the 3.5” bay with an SSD but leave the chip buried and untouched? Will the system continue to “fusion” (but with limited benefit as the SSD is probably nearly as fast as the flash(?) chip)? Or is there a way to tell the computer to just ignore the chip?

Thanks!

iMac 27″, macOS 10.14

Posted on Aug 30, 2020 11:22 AM

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Answer 1

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Kappy

Level 10

361,765 points

Aug 30, 2020 1:35 PM in response to Mike G.

The best information I could find is in 2012 only two Fusion Drive options were available - 1TB or 3TB. If yours is the 3TB model, then it came with a 128GB SSD flash drive or PCIe. Splitting a Fusion Drive is easy - How to split up a Fusion Drive | Macworld. I suspect that as soon as you replace the HDD, the Fusion Drive is split.

Both options include a discrete flash drive component but I don't know if they are pluggable blades or chips on the motherboard. Whichever they are, they are probably proprietary. When you split the 3TB option, you end up with a 128GB SSD (about 120GBs after formatting) and the 3TB HDD. The HDD can also be replaced by an SSD up to 768GBs according to the specs. I'm not sure if the size limit is due to Apple's hardware or that there weren't any larger SSDs at the time. Of course the SATA bus would limit the maximum speed of an SSD.

I also found these additional articles about fusion drives: set up a Fusion Drive and Splitting Your Data- An Alternative to Fusion. They are from the same source and are fairly old.

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Answer 2

Kappy

Level 10

361,765 points

Aug 30, 2020 12:17 PM in response to Mike G.

The SSD used in the 1TB Fusion Drive model is a small 30GB PCIe module. The larger Fusion Drive options came with a 128GB PCIe SSD. You need to see What to do if your Mac’s Fusion Drive fails (or shows signs of failure) and How to fix a split Fusion Drive.

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Answer 3

Mike G. Author

Level 2

421 points

Aug 30, 2020 12:58 PM in response to Kappy

Thank you. The disk is either a 3 or 4 TB model. (Because it is faulty I’m struggling to get a definitive answer.) Does that mean I have the SSD rather than PCI version of the solid state device? Any idea if that’s physically separate from the mechanical disk?

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Answer 4

Mike G. Author

Level 2

421 points

Aug 30, 2020 1:07 PM in response to Kappy

Thanks. These articles point to a possible explanation of what might the problem, but not how to solve it.

The second article explains how to “fuse” a SSD and a traditional disk to create a single fusion volume... but how do you do the reverse — i.e., separate a fusion drive into a separate SSD and traditional disk component? (I’d then simply not use the solid state component, and instead rely on the newly-upgraded main disk.)

-Mike

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Answer 5

samtenor

Level 4

1,720 points

Aug 30, 2020 3:10 PM in response to Mike G.

You probably will be better off with SATA SSD as boot-drive. Just leave the PCI-e (chip SSD) along, for data or Photoshop's scratch disk.

I was successful with 1T SATA-3 SSD -- and I have learned a hard lesson with "fusion drive" attempt between PCI-e SSD + SATA SSD. I had to use the internal SATA SSD as main drive.

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Answer 6

Mike G. Author

Level 2

421 points

Sep 10, 2020 9:46 PM in response to samtenor

Thanks everyone. For an update: I left the Apple SSD in place and replaced the Apple HDD with a new one (which happened to also be an SSD). After boot, I discovered, as you guys predicted, a second volume showed up which was the 128 GB Apple SSD. It’s not a useable volume currently, but I’m planning to format it and use for scratch.

Thanks for the advice!

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Fusion and hard drive replacement on late 2012 imac

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