Fusion Drives in iMacs: still worth it (or perhaps even MORE worth it)?

The most recent discussion on this topic I can find here is from last May.

I (based partly on Adam’s success replacing Apple’s tiny 28 or 32 GB “blade” SSD in the processor direct slot) did the same and repurposed the rotating platter drive in the drive bay to a place to store Time Machine backups (a fool’s errand, of course, if the iMac fails because of some problem that makes it unable to boot).

Currently I’m involved in a discussion on Mac-L with another list member there who is considering doing the same thing, and that discussion has gone into the weeds as regards the logical structure of Apple’s “Fusion Drives.” Apple seems always to have advertised the total storage as being equivalent to the size of the rotating platter hard disk, but I’ve been told on Mac-L that the two storage elements are logically additive. I’d assumed that the tiny blade SSD in the processor direct slot actually worked like a programmatically customized cache, storing at least the boot elements of the OS and data that’s used a LOT. However, given the decreasing expense of SSD 'drives" that would fit in the drive bay, perhaps using solid state memory in BOTH locations, e.g., OWC’s 1 TB stick in the processor direct slot and a 2.5" “drive-sized” 1 TB internal SSD in the drive bay and mating them into a 2 TB “Fusion Drive” would create a VERY nice upgrade, if indeed 1 + 1 = 2 rather than 1 + 1 = 1, but in 2 different places

Of course, if the SSD in the processor direct slot functions as a cache rather than additive storage, it might actually REDUCE overall performance.

So, my bottom-line question: does a 1 TB NVMe stick in the processor direct slot + a 1 TB SATA drive in the drive bay, if fused together, create a 1 or 2 TB storage device? Has anyone here done it?

And, for people contemplating such an undertaking remember that, after ripping off the display, then exenterating the entire machine, before you start reassembly, the only things left in the case will be the BT and WiFi antenna modules and the screws holding the monitor base to the case. it’s tedious, but when you’re done you can do your best imitation of Tom Hanks in Cast Away when he erupts in self-congratulatory joy (OK, he did have an audience of 1 soccer ball), beating his chest and proclaiming “I HAVE MADE FIRE!!!”

On the other hand, after you’ve done this once, successfully, you’ll be tempted to go back in for other tweaks or for other reasons. In my case, because I started with an 8 GB RAM machine, I also boosted RAM to 32 GB, and 18 months later I began getting kernel panics, which REMBER blamed on faulty memory. OWC replaced that quickly and at no cost to me, but I’d gotten careless and overconfident, so when putting everything back together I munged two of the MOST bulletproof reconnections (the fairly robust connectors at the ends of the WiFi and BT antenna cables that pin on to the Broadcom BT/WiFi board), and once I reapplied fresh blue painters tape to the bezels of the case (a nice upgrade for those who want to boast their spelunking skills) I discovered I no longer had Bluetooth! (Well, actually, I had very primitive Bluetooth—my keyboard sort of worked, but none of my Apple BT mice would pair, my BT headphones would try to pair but just make a few horrible noises and then give up).

Knowing what I’d done wrong, I looked into replacing the Antenna cables and discovered that Apple’s authorized repair recommendation was a replacement of the entire metal case, because the antennas themselves are fused to the periphery of the case. Fortunately someone else who’s done this stuff told me that with the assistance of a a pair of illuminated magnifying spectacles I might be able to wedge a tiny jeweler’s screwdriver into the cable-end connectors, which basically work like a tiny gold plated alligator clamp to grasp the pins on the WiFi/BT board and bend those tiny blades back into position, and (miracle of miracles) that WORKED!

(Chapter Next): all my Googling and Duck Duck Going on this topic, realizing with a mixture of dread and elation, facing the prospect of telling my wife that I just MIGHT “need” to retire my iMac and purchase an M1x Apple Silicon iMac as soon as they appear next spring, led me to the realization that the processor in my machine is SOCKETED, not soldered to the motherboard, and that some have actually upgraded the Quad Core i5 to a Core i7 processor. Those discussions feature lots of worry about whether the cooling would be adequate, whether the fans would be spinning raucously non-stop, etc., and my guess is that I won’t go there, but the MAIN reason for that is that the processors themselves are still VERY expensive, and the performance gains seem to be VERY modest.

First off, I think the question is moot if you have a 1TB SSD. Install macOS on the SSD and format the HDD as a separate secondary volume where you can manually store documents that you don’t want on the SSD.

With the SSD big enough to hold macOS, your apps and home directories, you really don’t need macOS to be automatically moving files between the two devices.

To answer your question, based on what I remember reading many years ago (when this was new tech):

  • Fusion drives formatted for HFS+ have a capacity that is the sum of both devices. A file only exists on one device and macOS moves files between the two according to the Fusion Drive logic.

  • Fusion drives formatted for APFS have a usable capacity that is the size of the HDD (maybe the largest drive, but it would be a pathological condition to set up a Fusion drive where the HDD is smaller). All files always exist on the HDD and the SSD acts as a cache.

    So even though the fusion drive will appear (to disk utilities) to have a capacity that is the sum of both devices, once you start writing data, the practical capacity will be that of the HDD.

That being said, take a look at the following article about Fusion drives: Fusion Drives in APFS – The Eclectic Light Company

When a fusion drive is formatted for HFS+, what you get is:

  • Two storage volumes (one on the SSD and one on the HDD) are bound together as a CoreStorage logical volume, which is presented to the system as a third (virtual) storage device.
  • This virtual storage device is then partitioned and formatted for HFS+.

When the fusion drive is formatted for APFS, you get something a bit more complicated:

  • The SSD has a single volume, formatted as an “APFS Physical Store Disk” and tagged as “Main”

  • The HDD has a single volume, also formatted as an “APFS Physical Store Disk”, but tagged as “Secondary, Designated Aux Use”

  • A single APFS container is created, containing both of the physical stores

  • Within that container, all of the usual APFS volumes are created.

    Since this article was written based on macOS Mojave (the first version where APFS fusion drives were supported), these contain your root, Preboot, Recovery and VM volumes.

    I suspect that if you do this today using Big Sur, that APFS container will have System, Data, Preboot, Recovery and VM volumes - like what you would find on a non-Fusion drive.

I’ve never done it, but …

With a genuine Apple Fusion drive (not sure if it will work if you’ve replaced your internal drives with aftermarket parts), the diskutil resetFusion command is supposed to do all the work for you (wiping both devices in the process). Here’s the text of the manual page from Big Sur (type man diskutil and scroll down a bit to find it):

If that won’t work for your system and you need to do it manually, here are some articles that may help:

The articles from 2012 only describe making HFS+ Fusion drives, because that’s all there was at the time. Note also that an APFS Fusion drive requires macOS 10.14 (Mojave) or later.

A summary of the procedure for HFS+:

  • Create a CoreStorage logical volume group consisting of the two drives you want to fuse
  • Create a logical volume on that group.

A summary of the procedure for APFS:

  • Create an APFS container spanning both devices, specifying the SSD as main and the HDD as secondary. This container will cause a new virtual device to be created
  • Add an APFS volume to the container’s virtual device

Allegedly, that’s all there is to creating a Fusion drive - a CoreStorage volume group or APFS container that spans multiple devices.

What I don’t know is if macOS will, upon seeing an SSD grouped with an HDD will automatically start implementing Fusion Drive logic (keeping most-commonly-accessed files on the SSD and everything else on the HDD) or if you’ll just end up with a dumb spanned volume where performance will vary based on where files happen to end up being stored.

I think we need information from someone who has actually tried this in order to answer that question.

But ultimately, I think this should be just an academic exercise for you. With a 1TB SSD, I see no reason why making a Fusion drive would be better than simply using it and your HDD as separate storage devices.


David, thanks so much for that. The possible use case I could see for making a Fusion drive containing one solid state device in the processor direct slot and another solid state device in the drive bay in an iMac would be if someone was doing lots of video editing that benefits from really fast real-time updating. Of course, if it’s the case that when the storage devices are formatted APFS, they’ll have an aggregate logical space the same size as the device in the drive bay, there’d be no point to the exercise.

Howard is an incredible resource. I’m off to read his article!

Thanks again,
Jim Robertson

The command ‘diskutil resetFusion’ in Recovery mode was the final escalated support move from Apple when my iMac (2017) got itself into a terrible mess during an upgrade from Mojave to Big Sur. It worked and I ended up with a nice clean HD. Apple did bother to check that I had backups of my files.

Just curious. Are you now able to tell whether the rotating platter drive in the hard drive bay is now formatted HFS+ vs. APFS?

Thanks so much,
Jim Robertson

I don’t know how I’d distinguish between the rotating platter and the SSD but Disk Utility labels ‘Fusion Drive’, ‘Container disk2’, ‘Macintosh HD’ and ‘Macintosh HD - Data’ as APFS, so I guess that applies to both?

If it’s a Big Sur boot drive, then the Fusion drive is definitely APFS.

You can easily check with the following CLI commands:

  • diskutil list - list all volumes (physical and virtual)
  • diskutil apfs list - list all APFS containers and their contents
  • diskutil cs list - list all CoreStorage volume groups and their contents