Fusion Drive Functionality Question

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Fusion Drive Functionality Question

I have a 2017 iMac which came with a 1 TB Fusion Drive. I replaced that original drive with a 4 TB Fusion Drive and have been operating for a couple years now, but I just noticed I may not be utilizing the SSD portion of my hard drive (see attached screen shot https://www.dropbox.com/s/q3x5m5e6g1l5f7t/Disk%20Utility.png?dl=0).

Disk Utility shows I have a 28 GB SSD Disk that is unformatted. Is that my built in SSD drive? Is it possible I'm not using the SSD portion of the drive? If so, is it safe to format that drive or will it format the entire 4 TB hard drive?

Currently running macOS Majave (10.14.5)

iMac (Retina 5K, 27-inch, 2017

32 GB RAM

iMac 27" 5K, macOS 10.14

Posted on Jul 18, 2019 10:24 AM

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Answer 1

Best reply

Craig Mclaughlin Author

Level 1

19 points

Jul 22, 2019 6:30 AM in response to raina_b22

Fusion Drive configured. System is flying along now!

The solution was I needed to format both drives then in Terminal allocate both drives to form a new virtual Fusion Drive.

Here's a good video explaining the process: https://www.youtube.com/watch?v=w4NqbGEJpN4

And the Terminal commands used:

1. diskutil list

2. diskutil cs create Fusion disk# disk#

3. diskutil cs list

4. COPY LOGICAL ID NUMBER

5. diskutil coreStorage createVolume PASTE_LOGICAL_ID_NUMBER jhfs+ "Name Of Drive" ENTER_SIZE_OF_FUSION_DRIVE

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Answer 2

Teddy_B

Community Specialist

Jul 19, 2019 7:09 AM in response to Craig Mclaughlin

Hello Craig Mclaughlin,

After reviewing your post, it sounds like you replaced the original drive and now you see a second disk that is unformatted. It sounds like you have a split Fusion Drive. I would recommend you use this article to help troubleshoot the issue.

How to fix a split Fusion Drive

Please don't hesitate to let us know if the steps resolved your issue or if you need additional assistance.

Thank you for using Apple Support Communities. Have a nice day.

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Answer 3

raina_b22

Community Specialist

Jul 19, 2019 2:45 PM in response to Craig Mclaughlin

Hey Craig Mclaughlin,

Thanks for the update. It seems like Disk Utility shows a 4TB rotational HD and a 28GB SSD drive installed, but Terminal thinks you do not have 2 disk drives.

One thing to do is make sure you have a backup of your data to keep it safe. Since one of the drives is not being recognized, erasing and reformatting may help. Follow the steps in the How to fix a split Fusion Drive article, then you can reinstall macOS and restore your data from the backup.

How to back up your Mac

Take a look at the "Which format: APFS or Mac OS Extended" section.

How to erase a disk for Mac

How to reinstall macOS from macOS Recovery

Restore your Mac from a backup

Have a good one.

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Answer 4

Jul 19, 2019 11:25 AM in response to Teddy_B

Thank you for your quick response, but when I enter the command "diskutil resetFusion" in Terminal, I get this response:

"Your computer must have exactly 1 solid-state and 1 rotational disk drive"

I have both a 4TB rotational HD and a 28GB SSD drive installed. Do I need to perform a fresh format on each and unmount before they can be joined as a Fusion Drive?

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Fusion Drive Functionality Question

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