Restoring time machine backup on external drive following internal fusion drive failure

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Level 1

8 points

Restoring time machine backup on external drive following internal fusion drive failure

My iMac 17.1 (27 in) internal fusion drive seems to have failed or at least is refusing to boot. I can see the drive on disk utility but both partitions are showing full and repair doesn’t fix it.

I have a Time Machine backup of my iMac on an external drive. I’ve been trying to restore that Time Machine backup on a 4TB Samsung T7 external drive without success.

Am I wasting my time, is it possible to do, how should I do it?

My Time Machine backup was saved under MacOS 10.15.7.

Under restore mode, I’m able to start the time machine restore. It starts, I see a progress bar then after some time, the screen blacks out and iMac becomes unresponsive to keyboard or mouse.

After 14 hours, once the Time machine HDD was quiet, I shutdown the iMac and it refused to reboot.

I used disk utility on the restoration destination drive and it is showing only 8GB used and 4TB available so clearly it didn’t restore my 2.7TB data

Any help would be appreciated.

iMac 27″, OS X 10.11

Posted on Sep 28, 2025 8:40 AM

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Answer 1

Top-ranking reply

Grant Bennet-Alder

Level 10

139,799 points

Sep 29, 2025 8:24 AM in response to archieluis

<<. I tried that and still encounter some issue. My iMac 17.1 is a late 2015 and the recovery MacOS is El Capitan. I can install it no problem on external drive and boot from it. >>

In the time between when your Mac was released and present day, the drive format transition from MacOS Extended to Apple File System APFS began and was completed.

if you have MacOS 10.11 Installed and working, then do not erase your MacOS. Use the full power of that working MacOS to install an UPGRADE. First apply any offered software updates. Then your Mac should offer you a later version.

if a later version is not offered automatically, use the links in this article to download a later version from the Mac App Store. You MUST use the links provided, because MacOS software is 'put away' in the Mac app store, and not generally available

How to download and install macOS - Apple Support

As you install a valid later version, the disk format will quietly be transitioned from MacOS Extended to APFS.

I would consider installing High Sierra first, as it is the oldest version that knows anything about APFS.

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Answer 2

Sep 28, 2025 8:59 AM in response to archieluis

In recent MacOS, the System is NEVER restored from backup. Even If you could manage to write the files, it would never boot.

The new "Approved Method" is to Install a new instance of MacOS, derived from fundamental sources, on the desired destination drive FIRST. This will build a new instance of MacOS AND a verifiable virtual system drive in a way that is bootable.

Then you can use migration assistant to import your files and settings from your backups.

------

Fusion Drive is an internal detail that does NOT interfere with this process in any way. At the level MacOS looks at things, your fusion drive is a Volume like any other Volume. How the Boot drive works internally and is maintained is not important to MacOS, and does not change how MacOS treats it in any way.

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Answer 3

den.thed

Level 10

90,400 points

Sep 28, 2025 9:04 AM in response to archieluis

Short version:

1) Startup in macOS Recovery

2) Format the external SSD (GUID/APFS)

3) Install macOS 10.15 Catalina on the external SSD

4) Startup from the SSD and Migrate from the Time Machine backup

see > Use an external SSD as your startup disk … - Apple Community

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Answer 4

-g

Level 4

3,365 points

Sep 28, 2025 9:12 AM in response to archieluis

imac17,1 that would be 2017 hardware?

did you Erase the external ssd (APFS format) and install/update a fresh macOS on it before using its Migration Assistant app to restore your User from your Time Machine back up?

all non-Apple branded hardware removed?

I work mostly with Clones (in the past), but are all your paswads correct in the process, is your Time Machine encrypted?

Can you use DriveDX and get the drive health and hours of your internal SSD/HDD (may offer a clue to drive failure)

even if you get the restore working on your external -- and your Fusion SSD/HDD reformated -- I would replace the HDD with a SATA SSD (and the PCIe SSD with a much larger drive)

an ACASIS TBU405 PRO Thunderbolt3 enclosure (and retail PCIe NVMe M.2 SSD) will deliver 2500MBs read/write on 2017 hardware if you go the external SSD route...I believe that is twice as fast as your T7 SSD and it allows you to easily swap SSDs...

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Answer 5

archieluis Author

Level 1

8 points

Sep 29, 2025 7:02 AM in response to den.thed

I tried that and still encounter some issue. My iMac 17.1 is a late 2015 and the recovery MacOS is El Capitan. I can install it no problem on external drive and boot from it.

my Time Machine backup is showing MacOS 10.15.7 which is Catalina

But I know for a fact that I was running MacOS Monterey before it stopped booting.

Now when I installed Catalina, it’s very unstable and restart by itself. Once I was able to get it, I tried migration tool and was unable to recover my Time Machine backup.

I tried to upgrade to Monterey but that failed and unable to boot.

So I’m back to square one and just erase and format my SSD again and will try again.

I already bought a MacBook Pro to replace the iMac, I’m just trying to recover the Time Machine backup in order to access files that I didn’t have time to transfer before the iMac crashed.

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Answer 6

steve626

Level 6

13,623 points

Sep 28, 2025 9:24 AM in response to -g

-g wrote:

imac17,1 that would be 2017 hardware?

iMac 17,1 is a late 2015 iMac and it can be upgraded to Monterey

an ACASIS TBU405 PRO Thunderbolt3 enclosure (and retail PCIe NVMe M.2 SSD) will deliver 2500MBs read/write on 2017 hardware if you go the external SSD route...I believe that is twice as fast as your T7 SSD and it allows you to easily swap SSDs...

The late 2015 iMac 17,1 has Thunderbolt 2 ports so it can't utilize the Thunderbolt 3 enclosure. Its USB ports can provide up to 5 Gbps. I have an iMac 2015 model and some years ago replaced its internal mechanical drive with an internal SSD.

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Answer 7

-g

Level 4

3,365 points

Sep 28, 2025 9:59 AM in response to steve626

steve626:

thanks that information about model year, I went from a 2010 Mac Pro to 2017 iMac 5K -- got it off eBay Grade A -- was advertised "flash" memory -- i didn't know what that was except it was SSD, but nearly fell off my chair when AJA timed its MBPs (1800-2400) I was expecting 500MBPs

ive been hooked on iMac ever since with their Retina screens -- 2017 is now long in the tooth for an only machine (still great for Mojave and Photoshop CS6 and FCP editing, though) -- I think 2019 will become the Intel classic for its upgradability and cheap parts...

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