Problems with mounting an external Fusion Drive to a 15-inch 2018 MacBook Pro.

So, I had a problem with the external Fusion Drive to be used for backup for my PC. But because of problems trying it on PC, I decided to get an actual Mac computer to see if it works fine. When I plugged the external drive with the HDD enclosure into the MacBook with a USB 3.0 to USB-C adapter, the icon of the external drive doesn’t pop up. When I go to Disk Utility, it does show the external drive, but when I click mount, nothing happens. No error, no sign that it’s working, though I think the read/write lights does seem it’s trying to work, but I think the latest macOS version got confused and doesn’t give me something what happened to it. I did try First Aid, but bizarrely it was complete, and it didn’t work. I even did the same thing with Recovery Mode with the same options, but they’re exactly the same thing. I even tried a restart of my MacBook to see if it detects the Fusion Drive properly while it’s booting, but that also didn’t work. I did try to see if my MacBook can boot to my Fusion Drive using Target Disk Mode, but it shows the Thunderbolt/USB symbol, aka it’s waiting for a mounted external drive I think, so I skipped. I did try disk recovery using Disk Drill, but even if the files were fully recovered, they weren’t actually complete, they’re partial. I don’t know it has something to do with the drive starting to become faulty for being used for a few years and it fails to boot, alongside the display from my 27-inch 2017 iMac due to the dust causing component problems, or maybe it was encrypted to only be used on the specific iMac. Disk Drill did still detect that this is an APFS drive though the partition for my stuff still says “RAW”, so maybe my Fusion Drive is starting to stop working in the next several months or years or so, hence why it’s unmountable. If anyone knows any problems or suggestions, let me know. Thanks. 🙂

MacBook Pro 15″, macOS 15.4

Posted on Apr 26, 2025 7:47 AM

Reply
Question marked as Top-ranking reply

Posted on Apr 26, 2025 8:35 AM

This sounds strange to me.


A Fusion drive is a combination of two pieces of hardware within the chassis of the iMac. It is a low capacity SSD and a higher capacity HDD. The two devices are coupled by software (fused) to work as a single unit. It's unusual to have any sort of "fusion drive" operating as external storage.


If this is a single external drive connected to your iMac, then it is either an HDD or an SSD. And if it is your intention to use this as external storage, it sounds to me like you simply need to reformat it properly.

This will erase any data already on the drive.

Erase and reformat a storage device in Disk Utility on Mac - Apple Support


Connect the drive.

Launch Disk Utility.

Click View > Show All Devices

Select the device, usually with a name ending with "Media", not an indented volume.

Click Erase.


• If you want to use this ext drive only with your Mac:

Select the GUID Scheme, select APFS Format, provide a Name (select a scheme first)


• If you want to use this ext drive with your Mac AND a PC:

Select the GUID Scheme, select ExFAT Format, provide a Name


Click Erase to complete the process.

Similar questions

4 replies
Question marked as Top-ranking reply

Apr 26, 2025 8:35 AM in response to sjb04

This sounds strange to me.


A Fusion drive is a combination of two pieces of hardware within the chassis of the iMac. It is a low capacity SSD and a higher capacity HDD. The two devices are coupled by software (fused) to work as a single unit. It's unusual to have any sort of "fusion drive" operating as external storage.


If this is a single external drive connected to your iMac, then it is either an HDD or an SSD. And if it is your intention to use this as external storage, it sounds to me like you simply need to reformat it properly.

This will erase any data already on the drive.

Erase and reformat a storage device in Disk Utility on Mac - Apple Support


Connect the drive.

Launch Disk Utility.

Click View > Show All Devices

Select the device, usually with a name ending with "Media", not an indented volume.

Click Erase.


• If you want to use this ext drive only with your Mac:

Select the GUID Scheme, select APFS Format, provide a Name (select a scheme first)


• If you want to use this ext drive with your Mac AND a PC:

Select the GUID Scheme, select ExFAT Format, provide a Name


Click Erase to complete the process.

Apr 26, 2025 9:32 AM in response to sjb04

I thought of one more possibility.


If this is a mechanical hard drive that used to be inside a Mac, as part of a Fusion Drive, and you have placed it into an external case, that might explain why you are having problems.


The mechanical hard drive is a part of the Fusion Drive, but it is not a complete Fusion Drive. If it was being used as part of a Fusion Drive, what it would have is a partial filesystem on it. The other part of the filesystem – and the one containing the pieces of some of the most-frequently-used files – would be on the SSD.

Apr 26, 2025 8:38 AM in response to sjb04

As far as I know, there is no such thing as an external Fusion Drive.


A Fusion Drive is a logical combination of a hard drive and a small SSD, created in software by macOS. There were Macs with Fusion Drives, but Apple never sold external Fusion Drives. Nor did they, to the best of my knowledge, ever give instructions on how you could turn two third-party external drives into one.

Apr 26, 2025 9:24 AM in response to sjb04

If you are by any chance referring to a SSHD, that is not a Fusion Drive.


A SSHD is a hard disk mechanism with a small amount of on-board flash (SSD) cache. It differs from a Fusion Drive in a few key respects:

  • The SSD storage is a pure cache – nothing else. With a Fusion Drive, there are pieces of files that live only on the SSD.
  • Since the SSD is a pure cache, and is located within the hard drive mechanism, its existence is transparent to the operating system – except for its effect on performance and performance tests. You can use a SSHD in a Mac, a Windows PC, or a Linux system. Fusion Drives only work on Macs (although there are other systems which have implemented similar concepts) because it is code within macOS itself that makes them appear to be a "drive".
  • SSHDs typically include MUCH less SSD storage than Fusion Drives do.

This thread has been closed by the system or the community team. You may vote for any posts you find helpful, or search the Community for additional answers.

Problems with mounting an external Fusion Drive to a 15-inch 2018 MacBook Pro.

Welcome to Apple Support Community
A forum where Apple customers help each other with their products. Get started with your Apple Account.