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User profile for user: jus10g
jus10g Author
User level: Level 1
12 points

Late 2015 27” iMac: replace dead fusion drive with external ssd?

The Fusion drive on my late 2015 27” 5k iMac fried last night. I’m pretty sure it’s the SSD half - I went into Recovery Mode but I couldn’t get the SSD to repair (problems found with partition map) or even erase (couldn’t write to last block). The HDD part of the drive appears to be fine. I have a recent Time Machine backup, so my data is safe.


Im wondering if I can just buy an external SSD as a boot drive and then continue to use the internal HDD for storage. My questions are as follows


1) Does anyone have any experience running in this configuration externally? Or should I get someone to do the work to do an internal replacement?


2) Do I need to replace the storage HDD with an external drive as well? Or can I continue to use the internal storage?


3) What SSD should I get? Size (512Gb?)? Connector? (Thunderbolt or USB 3).


4) Is Apple Data Migration/Time Machine smart enough to figure out how to split up my backups into the 2 new drives? How do I get back up and running again?


This community has been incredibly helpful and generous with time and advice in the past. Thank you in advance for your help.





iMac Line (2012 and Later)

Posted on Sep 17, 2019 2:29 PM

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User profile for user: macjack
macjack
User level: Level 10
111,204 points

Posted on Sep 17, 2019 2:39 PM

  1. If you want to run from an external SSD you can set that up yourself easily. But be aware you will not get the same speeds as if it was installed internally.
  2. You can continue to use the internal for storage but you'd be better off putting everything on the SSD.
  3. I would install an internal SSD and have Apple Authorized Service Provider to the job, if you are not comfortable with hardware.
  4. You can install the system on the external (if you decide to go that route) and then Migrate from the HDD.

How to move your content to a new Mac - Apple Support

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User profile for user: macjack
macjack
User level: Level 10
111,204 points

Sep 17, 2019 2:39 PM in response to jus10g

  1. If you want to run from an external SSD you can set that up yourself easily. But be aware you will not get the same speeds as if it was installed internally.
  2. You can continue to use the internal for storage but you'd be better off putting everything on the SSD.
  3. I would install an internal SSD and have Apple Authorized Service Provider to the job, if you are not comfortable with hardware.
  4. You can install the system on the external (if you decide to go that route) and then Migrate from the HDD.

How to move your content to a new Mac - Apple Support

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User profile for user: Kappy
Kappy
User level: Level 10
361,849 points

Sep 17, 2019 2:47 PM in response to jus10g

1) Does anyone have any experience running in this configuration externally? Or should I get someone to do the work to do an internal replacement?


Nothing special is needed. You only need to partition and format an external disk and install macOS.


2) Do I need to replace the storage HDD with an external drive as well? Or can I continue to use the internal storage?


You can continue to use the internal HDD, if it is working.


3) What SSD should I get? Size (512Gb?)? Connector? (Thunderbolt or USB 3).


Whatever best suits your needs. Your iMac model supports USB 3.0 and TB2.


4) Is Apple Data Migration/Time Machine smart enough to figure out how to split up my backups into the 2 new drives? How do I get back up and running again?


No, it isn't. Furthermore, it is best used to restore a working system volume and/or Home folder. It's nearly impossible to fully answer this without knowing what disk replacements you will decide to get. The easiest way to get things started would be to partition and format the external disk then install macOS so that you have a functioning startup volume. Once you get this done, then you can tackle the rest of the task.


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User profile for user: jus10g
jus10g Author
User level: Level 1
12 points

Sep 22, 2019 8:38 AM in response to jus10g

I bought a Samsung t5 1tB SSD and installed Mojave on it. I'm able to boot off of it on the iMac (via USB3), but the performance is very laggy. It appears the OS is getting hung up on trying to mount/load the internal drives. If I try to click through to the internal drive using Disk Utility or Finder, I get a spinning beach ball. I went into Terminal and tried several different approaches with diskutil to repair or erase the internal drives, but the system kept getting hung up being unable to unmount. I even tried changing etc to prevent automounting of the drives, but that didn't work either.


So I don't see any way around having to open up the machine to remove/replace the broken Fusion drive. I'd like to keep my prior system on a single drive, which would mean getting a 3TB or 4TB internal HDD drive. But from what I can tell from reading around, for heat and performance reasons I'm better off replacing the Fusion with a 1TB SSD boot drive and then getting some external storage solution for all my files (music, videos, applications, documents).


The iMac drive replacement looks finicky, so I'll probably take this to a local shop (near Oakland, CA) to do the work. If anyone has any suggestions on specific drives (internal SSD boot or external storage), I'd appreciate it. Thanks.

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User profile for user: samtenor
samtenor
User level: Level 4
1,786 points

Sep 22, 2019 3:05 PM in response to jus10g

I found the "Fusion Drives" tend to be tricky when issues arisen.

I went through similar experience and had better manipulation when I used MacOS USB boot drive for repairing (still no idea why).

In my case, I replaced HDD with SSD and had better success when both drives were formatted (via USB Boot Drive) with "Mac OS Extended" -- as the format seems to be more accessible for both disk utility and terminal.


I would suggest to stick with 1 single drive SSD (even just SATA 3) -- to avoid any issues of future ugly Fusion Drive "divorce" battle.

In your case, external SSD did not perform to your expectation, that internal SSD might be your next choice. OWC carried various of SSDs (I never used them -- I am a Crucial fan) with good reviews. I believe there might be 2T SATA3 SSDs around.

Whatever you do, a good back up is essential, and I just found out one of my TimeMachine drives also kicked the bucket. Good thing every Mac in my hous, has at least 2 TimeMachines.

Therefore, extra "TimeMachine drives" or/and "external data drives" might be another suggestion since you have such vast data.


Sam

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Late 2015 27” iMac: replace dead fusion drive with external ssd?

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