Fusion Drive Half Dead

Didn’t see that coming - but the HDD portion of my Fusion drive in my iMac 27" 5K retina (late 2015) died last Friday. Thanks to bootable backups I didn’t miss a beat for work… but now it reality sets in. What to do what to do. Normally I would crack an iMac open and replace a drive with an SSD in under 30 mins… but reading the iFixit on these models looks UGLEE… so I have opted against that option and having it replaced is cost prohibitive, so I am weighing my options and was hoping for some input.

Here’s where we are at:

  • iMac 27" with split FUSION drive - 128G nvm(?) blade SSD still working great (1gbps read/400mbps write)
  • New external 1TB SSD Thunderbolt 2 (400mbps/400mpbs USB-C to USB - does ANYONE know where to get a USB-C to TB2/MiniDisplayPort cable for this could be found)
  • Several other external HDD used for cold storage

My initial thought was system on the 128G SSD, applications, your basic system, but relocate my home and work directory to the external SSD. It worked, at first, then one after another apps wouldn’t open, or opened but couldn’t open prefs, etc… all having to do with the home folder located on an external drive. I tried to find solution for this but after a few hours I decided it wasn’t going to happen.

So now I’m back to a fast system loaded on the 128G SSD and part of my home folder restored (Migration Assistant/Carbon Copy Cloner) to the 128G, but the drive of course is now packed.

I do have the external SSD bootable but with only 400mbps throughput its not really usable for work, i bought it thinking the TB2 cable would not be that hard to acquire but I was wrong. I hate to leave that fast SSD sitting in the iMac as well (although it could be argued it is reaching it’s lifespan).

I have erased and restored more times in the last week than I care to mention and I’m getting tired of looking at Carbon Copy Cloner interface instead of making money. I was hoping someone might have some insight or suggestions what to do. Kinda wondering if I could fuse the external SSD and internal SSD… hmmmm

Without cracking the case and installing a SSD in the faster internal bus, that external SSD is your fastest alternative. Personally I don’t find it too bad, especially since you can keep the OS on the internal 128Gb SSD.

People have attempted creating dual-SSD Fusion drives but I consider it a potentially dangerous kludge, and one that apparently is unstable.

FWIW I usually recommend that people buying Mac desktops get AppleCare+, then replacing the hardware after the 3-year extended warranty expires. It not only protects you if something like this happens within the warranty period, it means you’ll always have a machine under warranty, you’ll get decent resale value from the old hardware, and you’ll always be up-to-date on processors (and especially ports - you’d have USB-C/TB3 with a current model, for example, which would offer very reasonable speeds for external booting).


I am trying to keep clean install system on the 128G SSD from old fusion, but my problem with my home folder. Even trimming considerably its still jammed packed in. I do a lot of development and I also work on this computer so my home dir is a bit large.

Why is it I don’t seem to be able to mount another drive as my home directory without causing mass permissions issues? The external SSD is plenty fast for my home, I just can’t get it to work right. It is even sanctioned by Apple.

I would check with an authorized Apple service center. It’s been about 3 years since the last time I had a drive replaced, but they only charged $100 + the cost of the drive.

Wow that’s not that bad will have to check thx

I would definitely see about having the drive replaced by a professional.

You should be able to have your home directory on an external drive. I did that several years ago and it worked fine. How did you move your home directory? Here’s how I would have done it:

  1. Create a second “admin” account via System Preferences.
  2. Log out of my personal account
  3. Login as the new “admin” account
  4. Open System Preferences > Users & Groups and then control-click on my personal account to bring up the Advanced Preferences.
  5. Change the Home Folder there

Is that what you did? I would recommend always having at least one admin account whose home directory is on the internal SSD, just in case your external drive dies completely, you’ll still be able to log in.

I cannot recommend a dual SSD Fusion setup on a computer that you earn your paycheck with. Interesting concept… but not reliable in any sense. Moving your home folder to an external HD will probably impact your means of making money in a negative manner. If this was a personal computer and you didn’t mind tinkering with it most of the time, then I would encourage you to go the less reliable route of what you are trying.

Since Apple could replace your dead spinning HD and pair it back to the SSD for around $200US, wouldn’t this be a better option? Or have an AASP perhaps install a larger spinning HD for around the same cost.

Fusion drive systems sound like a great concept in theory. You get SSD speed for the OS and cheap storage for the rest. Problem is the spinning HD is going to fail sooner than we want it to.

1 Like

I agree completely, but two years ago I went with a Fusion Drive anyway for the additional internal storage in my Retina iMac (plus AppleCare+ plus BackBlaze plus several Tb external storage all backed up and Backblazed). The hardware is 27 months old, so I’m planning to replace it by the end of the summer (and AppleCare+) with an iMac or possibly iMac Pro.

Although the street and Apple prices of SSDs have fallen considerably in the last few months I’ve had such good experiences in speed and utility with my Fusion Drive I’d consider getting it again. The iMac Pro does not offer a Fusion Drive option but iMacs continue to: the top-tier iMac I’d currently consider comes with a 2Tb Fusion Drive, though the FD ‘upgrade’ is an outrageous $100 for merely an additional 1Tb HD storage. (Nor is the $700 internal 2Tb SSD option a good deal.) With a $250 Samsung external 2Tb SSD using USB Type-C, I’d be perfectly happy continuing to use an internal Fusion Drive with external SSD storage, as long as I continued to maintain current backups in case of problems within the warranty period.

1 Like

For future reference - I must have spent 10 hours going in circles looking for solution. It is rooted in the new READ-ONLY APFS volumes. They are created in Catalina without most people knowing and then magically mapped over existing locations so you never really notice. The jest of it is - the read-only drive keeps the system safe. Problem is when you start trying to move your /Users directory to an external drive mapping the external /Users over the smaller system using ftsab method - you need to be able to move the Users dir and replace it with a symlink. When the system boots with the external users drive attached - something is being written to that READ ONLY drive. It may be something as simple as checking the readability of the Users dir, either way the READ ONLY drive part boinks it. s soon as you disable the protection (csrutil…) you can map users drives directly to a symlinked Users that list on the boot/system drive. I have yet to turn the protection back on to see if it works still after initial setup.

FWIW - I tried desperately to use the Users & Groups > Advanced method to move my Users folder and it worked fine, but way to much of my stuff was broken. Much of my automation was broken, but that same automation works fine when I map drive over the Users symlink.

I would not fight that hard against the OS.¹

I would leave /Users/ alone and have one admin user whose $HOME is in /Users/ and then simply change the $HOME of other users to be /Volumes/Users/ or whatever.

That way if my /Volumes/Users/ was damaged or failed somehow, I could still log in to my admin account with a regular /Users/-based $HOME.

In fact, given that APFS now lets you make these volumes that share the disk space of the main volume, you can have all the benefits of partitions without the downsides, such as getting stuck with a partition that was too small or too large.

I’ve already started moving 3rd party apps to /Volumes/Applications/ where possible, which means that I can share them easily between Catalina and Mojave (I am currently dual-booting my MacBook Air).

Of course, some apps are installed via .pkg and cannot be installed there, and Mac App Store apps insist on being installed in /Applications/ and I’m not moving those.

I thought about moving my $HOME to /Volumes/Users/but I suspect there are probably more potential problems sharing a $HOME between OSes than applications, so I decided against that.

¹ Obviously you‘re welcome to do as you wish, but it’s not a route I would recommend.