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I've been getting the message that my Mac has run out of application memory.
I've been trying to clear up some disk space, but some how it doesn't move free space up, in fact it decreased.

Then I saw this article https://support.apple.com/en-gb/HT207584 that says:

If your Fusion Drive appears as two drives instead of one in the Finder, it's no longer working as a Fusion Drive.

Is the SSD below what the quote is referring to?

Don't recall seeing this Container disk2 thingy around before.

Should I follow the steps in the support article to merge the drives back? I would prefer an option that doesn't wipe all my data.

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Update:
The article is referring to the Storage tab from About This Mac.

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So does this mean my drive is fine? I just need to continue clearing up more stuff?

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Note that the article says "in the Finder". Disk Utility shows you more detailed information, because it's ... well... Disk Utility. The Fusion logical volume is stored within a "container" data structure. You may see more information if you have checked "Show All Devices" in the View menu.

A fusion drive is usually made of 1Tb mechanical drive with a 120Gb SSD. If your Fusion drive had "split", you would see two separate volumes on the Desktop.
(Oh, and you wouldn't have a working system, as both volumes would be unreadable and all your data would be lost!)

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Should I follow the steps in the support article to merge the drives back? Would prefer an option that doesn't wipe all my data.

No! Don't do this! Your Fusion Drive hasn't split!

I believe you're confusing two issues: running out of application memory and your fusion drive becoming disassociated.

As for the first issue, see the post “Your system has run out of application memory. How is that possible?” for further details, but to summarize it, you've run out of RAM and swap space on your drive.

A Fusion drive is nothing more than Apple's incarnation of what the industry calls a hybrid drive. Basically, you're combining an Solid State Drive (SSD) with a Hard Disk Drive (HDD) to get the advantages of the SSD, but with the affordability of the traditional hard drive.

What you're seeing in Disk Utility is the Fusion Drive (Apple doesn't show the individual physical drives there), the APFS container and then the "Macintosh HD" partition within that container.

If you issue the command (in Terminal) diskutil list, you should see at least 3 entries - disk0, disk1, and disk2 - the SSD, the HDD, and the combined Fusion Drive.

Looking at disk2 you'll see something similar to below. Note it says it's a Logical Volume on disk0s2 and disk1s2. Those are the two physical disks/partitions that make up the Fusion Drive.

$ diskutil list disk2

/dev/disk2 (internal, virtual):
 #:                       TYPE NAME                    SIZE       IDENTIFIER
 0:                  Apple_HFS Mac HD                 +1.1 TB     disk2
                             Logical Volume on disk0s2, disk1s2
                             8B75BB26-04B0-4BAC-92A3-422A452CDAF1
                             Unencrypted
  • Yes, @resting, do not do anything to your hard drive. Normal hard drives like SSD's or Disk Drives will also have a "Container" that will contain your "Macintosh HD" Volume. You can also run the following command to better understand what is inside your container: diskutil apfs list – chrisdahfuh Mar 1 at 18:24
  • Point of order: Fusion Drive is NOT a hybrid drive. Hybrid drives use SSDs as cache, duplicating frequently used data from the HDD. Fusion Drives are tiered storage: the OS manages which blocks go to which part of the device. – benwiggy Mar 1 at 19:14

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