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A2115 / 2019 / Processors from 3.0 GHz 6-core i5, up to 3.6 GHz 8-core i9. Released March 19, 2019.

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Ending Fusion-Drive with SSD upgrade, while keeping existing macOS?


My iMac 19,1 is painfully-slow, due to it's Fusion-Drive. I have read a lot on these and other forums about disconnecting Fusion-drives, to replace the PCIe/NVMe and/or HDD with new SSDs (PCIe/NVMe + SATA).

I've read that it's possible to disconnect the Fusion Drive either by booting from a USB media, or Time-Machine backup, or by being fast at POST and, invoking the Recovery Console, finding the exact designation of both disks, and then using this and following the command-line syntax to 'break' the Fusion apart.

Then the iMac can be opened up to replace the HDD with an SSD most commonly, sometimes the PCIe/NVMe. I have read that is you are only replacing one drive (or want to dismantle the Fusion drive but keep the same hardware), then there can be issues with the Mac trying to 're-fuse' (boom boom), these peices of hardware - so the manual decoupling of the Fusion drive is important. Whereas, if swapping out both drives, then clearly there is 'nothing left' to work with, that the Mac would now work with.

A curved-ball suggestion seems to be running a new PCIe/NVMe drive in an external enclosure over Thunderbolt 3 as one's bootable macOS drive, potentially leaving the existing SSD & HDD where they are (but decoupled as a Fusion Drive), to just become 'some storage'. I think this setup would be fast than anything bootable that incorporated a replacement SATA SSD for the SATA HDD, as the limitation is SATA.

I think potentially, the 'external enclosure with the PCIe/NVMe drive Thunderbolt-3, may even be comparable to a motherboard-seated NVMe? As, what I understand overall is that the read-writes on 'whatever SSD, on any interface', maybe the more limiting factor, than any specific interface. So 'ultimate speed'sounds like it therefore comes from 'any' RAID-array of NVMe drives, in an external enclosure over Thunderbolt 3. Some has even created a RAID between a motherboard-mounted NVMe and one in an external enclosure! - that blows my mind, and I have no idea 'how'/what would be efficiently controlling the overall data read-writes, so the whole thing is balanced for speed/volume/interface!!!

So yeah 'All That'. BUT ...what scares me though, is that EVERYONE (bar one, @SirLuc ) casually refers to '...and of course you do a reinstall of a fresh macOS partition.

There is no 'Clone the bootable macOS partition spread across the Fusion drives, swap whichever drives one fancies, and then 'import' that Clone to (by default) the new, motherboard-mounted MVNe.

Why? As, surely the the purpose of Cloning is to potentially do EXACTLY this, with dissimilar hardware. I cannot beleive it is always 'easier' for every single person doing an upgrade, to start-over from scratch.

Yes, I know in principle 'it's better' to start afresh - things may run faster, for a brand new OS, more stable, for 'having less going on', and for all I know the overall it may be 'faster to do', even having to download the whole Adobe master suite, re-install 1password to get all my passwords back, etc etc.

But what if I just like 'how it is', and want to 'keep it that way'? So for all those who are like-minded, but as far as I can tell have 'never asked' (or at least 'here', and been answered'!), what I want to contemplate is:

  • Cloning the bootable macOS partition to a new, large, fast NVMe drive in an external enclosure, via Thunderbolt.
  • ‘Hope’ that this drastic change in underlying storage hardware/array from PCIe/NVMe on the motherboard + SATA SSD. Vs PCIe/NVMe via Thunderbolt NVMe does not crap-out the macOS logical partition.
  • i.e. I could boot the Mac from this Thunderbolt-connected SSD and it would retain all my functional software, subscriptions etc. And I would see a clear speed-bump. Can I ‘test this’, while the pre-existing Fusion-Drive remains intact?
  • Ideally, to then be able to dis-associated the internal Fusion Drive into the NVMe SSD and the HDD - but, what it ‘left’/what does one ‘get’, for doing this? Is the motherboard-mounted NVMe still bootable, and the HDD become visible as a ‘data-drive’, or is everything ‘lost’? I believe it cannot be re-Fused, so it would be nice to know what the ‘result’ of disassociating the Fusion drive ‘looks like’?
  • Ideally, then to disassemble the iMac, remove the existing small NVMe SSD and replace with with the not from the Thunderbolt-connected External enclosure, onto the motherboard. Would it ‘plug-and-play’? Is a cloned, bootable macOS partition ‘hardware’ agnostic enough not to care either about the change in storage hardware, and interface?
  • Ideally, to replace the HDD with an SSD. Either, as a separate data-drive, or possibly a bootable Windows drive.
  • Can a Parallels VM be spun-off like this? Or, can a physically-separate Windows SSD be set up as ‘Bootcamp’, so bootable separately, AND be accessible from within macOS via Parallels?

However - I am not clear that Time Machine, Carbon Copy Cloner or GetBackUp can 'confidently' achieve any, some or all - of the above.

It may be possible though, as I think @SirLuc 's post in reply to one of the 'Fusion-drive replacment' type articles, says so. He used a bit of software I've never heard of - Superduper - to 'export, swap-out hardware, import back', and it seemed to work He/She.

I think this was 'only' Cloning the HDD part of the Fusion-Drive and 'making that work' however - with a side-helping of succeeding in porting a Windows partition care of WinClone too.

So that's my challenge - how to do this'yeah, be gone Fusion-Drive' hardware changing out stuff, without a fresh install of macOS.

OR if there is a 'perfectly merge everything' option that builds on a fresh install and 'grabs' from a 'backup' or a 'clone' then fine - but I cannot confidently 'find it', this far down the rabbit-hole.

Thank you!


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Выбранное решение

First a Fusion Drive is nothing more than a small SSD cache drive which holds the more requested data from the larger HDD. If you where to remove the blade SSD your system would still run! Just using the HDD alone.

So now lets talk about cloning... We don't clone anymore! What worked in older OS9 and early OS-X days is not effective with booting drives. If you had a external data only drive which was setup with HFS+ then cloning it to a second external data drive wouldn't be an issue! But Apple has moved on to APFS and its a radically different file system structure! And boot drives are more than an OS, apps and data! I haven't used cloning in over 12 years now of fixing peoples systems.

OK, so cloning is out how do I get my stuff moved over to a new drive? Apple supplies the needed tools within the OS! Both within the OS installer as well as an external application: Migration Assistant.

So this is the needed actions:

  • First run Disk Utility Disk First-Aid function to make sure the drive is in good shape
  • Make a full fresh backup of your drive, I would use TimeMachine with a fresh external drive
    • Now the wrinkle! Dual boot or BootCamp installs forces a double backup! You need to treat each OS space independently or use a backup app which runs within each space or has the ability to span the spaces. If you where still running macOS Sierra (10,12.x) which is the last version of MacOS using HFS+ then I can say these dual running backup apps work! But since you are running APFS make sure you talk with the vendor before you assume it will work (even that may not be enough as the support folks are not always clued in)

Now depending on what your version of macOS is you may have the ability to break the Fusion Drive set if that is all you want to do. Frankly, I've stopped doing that as often I'm replacing both drives so it doesn't matter. Apple has changed things a few times so you need to be careful you use the correct method otherwise you may not break the linkage thinking you had.

Now its time to open the system and replace your drive/s follow these guides:

BootCamp or Parallels? There is no easy answer here as it has a lot to do on what your workflow is and what your aims are. If you have the need to move data between the two environments then BootCamp is the better for many. But as we discovered backing up is a pain! Many folks use an external drive which they either boot up with under Windows or use a VM. I recommend that approach as one of the issues we have discovered is Windows is not as safe from malware as macOS. Apple has Gatekeeper within the systems firmware which protects both the core firmware as well the boot drive from the nasties. Sadly some malware will get around this if they are running on the system as well as the protection is only active within macOS. Having Windows running on its own drive helps.

And of course! Make sure you have and maintain a good antivirus and malware app. I personally run two just within my Mac's and my major work system (I'm a photographer) is physically gapped from the internet.

Getting the most out of your system, an internal PCIe/NVMe blade SSD is by far the best drive to run your OS from, as well as your major apps. Leaving a good portion of the drive empty! That gives both the OS and your apps breathing room for their Virtual RAM, Caching, and scratch Space.

Depending on what your workflow you might use the drive for your current project. Then using your SATA drive as the data drive for your completed work.

Many Pro's use external drives for their active work (over Thunderbolt-3 in your case).

Becareful! USB-C connections doesn't tell you if you are connecting at TB3 (40Gb/s) or just over USB protocol which is a slower connection only giving you 10Gb's! Thunderbolt 3, USB 3.1, USB Type-C: Making Sense of Connections

What is on the other side can also matter, again what your workflow is makes a difference! I work with very large images so my needs are better served by a RAID drive set, a video or music creator will likewise benefit from a RAID.

But! A writer or artist won't need that level of performance. On the other-hand someone doing animations or heavily layered object work would like architects!

That only leaves gamers, they have little need for RAID but using the internal PCIe/NVMe drive makes sense.

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iMac Intel 27" Retina 5K Display 2019 Hard Drive Изображение


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Hi Dan, thanks for all this info - very good. Drilling down into the tiniest details therefore:

∙ What is the max data-transfer rate that the motherboard NVMe drive on the 19,1 iMac can handle? - Gen 2/ 3? 4? - in terms of blade SSD to buy? I have the OWC options given that Apple's socketry is bespoke, but wanted a sense of whether it's complete motherboard, or SSD-limited.

∙ Leads to the question of whether this interface is any faster than the full-fat TB3 interface such that I could simply have a macOS / Windows install on one or two external NVMe SSDs on those ports and not even open the case... I cannot find relative 'interface speed' data as applicable to iMacs. Just glad I have a 2019 model without the motherboard-soldered SSDs of the 2020 spec!

(I am assuming one can set an iMac to boot from an external SSD in this fashion. I will also 'de-fuse' the Fusion Drive, as any SATA-involvement with the booted-parition would seem to be asking to slowwww overall processing)

RAID - If the speed-limit of the I/O on an SSD is the interface (PCIe, TB3, SATA) then is that not the bottleneck?

∙ A RAID of SSDs in an external enclosure could never exceed the speed of the connecting TB port. Does higher speeds = software-RAID, e.g. two physically separate SSDs in enclosures, each occupying a separate TB3 port? Could a RAID be btwn a motherboard SSD and one on TB3 - is this desirable?

Time Machine - Does it really 'reproduce' every installed app, password/installation keys, etc, from the backup, onto the 'new macOS partitions'?

∙ Last bit - I have a Windows VM only now. I had to pull an initially Bootcamp partition into MacOS using Parallels, to save space/speed. I will have to see whether a restored Parallels VM imported back to the 'new configuration' post-upgrades results in something funcational. If 'not good enough', then I will have to try WinClone or similar to make a copy, and see if I can put this 'over the top of' a fresh Bootcamp Windows install, on a separate drive...


@doctorcloudbase - What is the max data-transfer rate of a PCIe/NVMe interface? Give this a read The Ultimate Guide to Apple’s Proprietary SSDs Your system offers a four lane PCIe 3.0 interface running NVM].

A TB3/4 interface is technically dual pipe Intel - What Is Thunderbolt 4? (and TB3 So while we state 40 Gb/s its only 20-20 like a highway is bidirectional! So moving a large data blob from A to B will use 20 Gb/s for the move and the other side will offer the needed pathway from B back to A for the Ack's/CRC's. It also allows a dual move! So you could have two external drives where your system is saving to one drive (A to B1) and at the same time you could be reading from a second drive (B2 to A)

RAID - is a radically different approach of data storage and really only addresses the drive side of the equation. All drives have limitations! SSD's are the ideal choice over spinning Iron if you need speed. HDD's have the benefit of being cheaper! And many believe HDD's are better for long term data storage. RAID is always described as storage within a single storage point (frame/box). You also can't do RAID across dissimilar drive types.

The lines are getting blurry! TB3/4 allows dual attachments so having two cables from the external storage point to your system allows the benefit of two data flows but only a few systems offer this and its EXPENSIVE! I doubt you need that level of performance given you have an iMac Vs a 2019 Mac Pro.

TimeMachine - YES! You will still need to validate the licenses for many 3rd party apps. Your Apple Apps keys are stored within your keychain so your Apple ID login credentials will unlock the apps.

As I explained Window spaces are unique to windows volumes. I would recommend to talk with Parallels.


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