iMAC with a fusion drive - What is the largest SSD that can be used effectively on a Fusion drive (assuming the hard drive is 3 TB)

One thing no one has yet mentioned is that the PCI-e card

is an Apple custom PCI-e and the generic PC blades will not

work.

As the others have stated, setting up a Fusion drive with a large

SSD would be a waste. It is better to install system, apps, and put

"work in progress files" on the SSD and just use the HDD as either

a backup or data archive.

I agree with @Keith Barkley that getting a large SSD for a Fusion Drive setup is a waste. The SSD is only meant to cache the transfers to make the system seem fast when using a slow drive for the actual storage which is why Apple uses small 32GB SSDs in later Fusion Drive setups.

If you are going to go to the trouble of replacing the SSD anyway why not just get two SSDs since they are so inexpensive and replace the slow and possibly worn out hard drive since it is such a difficult repair? You may even end up with more usable storage by doing it depending on the size of the SSDs you purchase (assuming you need that much storage).

You might want to look over this article before you try and replace the SSD, assuming you even can.

https://www.macworld.com/article/3441837/what-to-do-if-your-macs-fusion-drive-fails-or-shows-signs-of-failure.html

At some point, it is just better to have two drives: the SSD and the rotational. Once you get to a 1 TB SSD, it really does not make sense to combine them as a fusion drive since most of the stuff you need easily fits on a 1 TB drive.

I'm giving 2 huge thumbs up to this answer.

When I read Apple was reducing the size of the SSD for Fusion drives I was shocked...until I used the newer system and didn't notice any significant difference between it and my own older Fusion drive.

If I were going to all the trouble to open up my iMac I wouldn't install a larger SSD for the Fusion drive, I'd definitely replace the HHD with a large capacity SSD and maybe I'd replace the original SSD with a larger one. But I might just leave it as is (I have the larger 128GB SSD). In either case I'd not bother with Fusion. Today I'm booting that iMac with an SSD in a USB 3 enclosure and it is faster machine now than when I was booting from the internal Fusion drive.

On Late-2013 iMacs that shipped with only an SSD, there is no traditional drive tray, and the SSD is buried beneath the motherboard, making it a science disassembly project to even get at it. I suspect that this is where the physical SSD is also located for a Fusion drive equipped 2013 iMac, in addition to the traditional drive tray and rotational device location.

In addition to whether one can actually accomplish building a fusion drive with a larger SSD via software, it remains how much patience and value you place on your time to do this. It might actually cost less to purchase a 2.5 inch to 3.5inch drive adapter, and replace the fusion drive with a n-Terabyte SSD. The performance gains would be quite noticeable.

I actually use a 1TB Crucial SSD in an external USB3.1 Gen 2 enclosure connected to my Late-2013 iMac via USB3, and Catalina appears as fast from this external solution — as Mojave does installed on the internal 250GB SSD.

You're welcome. I posted the link because it made some recommendations. Personally I have no idea if there is any advantage in using a larger SSD.

woodmeister50 wrote:

One thing no one has yet mentioned is that the PCI-e card

is an Apple custom PCI-e and the generic PC blades will not

work.

Thanks for pointing out that very important fact. I did think of it when I first read the thread, but completely forgot to mention by the time I had finished writing my post. Just to clarify for the OP and others: OWC does make a replacement SSD that will work with the proprietary Apple PCIe SSD connector and OWC officially provides support. It may also be possible to use a standard M.2 SSD with an adapter although no one will officially support the configuration so a user will be own their own to troubleshoot the problem.