Reading this article was like deja vu, I went through almost the same process last fall. I had purchased a used 27" 2017 iMac (3.8 GHz quad-i5, 40GB, Radeon 580) that came with a 2TB fusion that the previous owner has split. Was my first time with a fusion drive, so didn’t even know they could be split into separate devices. Was easy enough to re-join them since I didn’t have any data on the machine.
Being a fairly high-end machine it was still very usable but not as responsive as my old MBA with an SSD I was replacing. I was thinking about how I wanted to back up that 2TB drive when I had a different idea. I decided to upgrade to an external SSD as my main drive, and use the internal Fusion drive as an cloned bootable backup. I had used a waterproof external (Silicon Power Armor A80)* for work in the past for my startup drive so I wouldn’t have to bike with a laptop, so this setup was familiar/comfortable for me.
Since this would probably be my startup drive on my main computer for the foreseeable future (easy to migrate to a new mac by just plugging in an external), I decided to go for the higher performance of a NVMe drive as well. I went with the WD Black SN750 1TB drive in a TEKQ Thunderbolt 3 enclosure. I also looked at SanDisk’s Extreme Pro drives, but the price was higher and performance was lower.
The enclosure does get warm, but not hot, and I have it resting in the Vesa monitor mount in the back of the iMac. CCC does a nightly clone to the internal fusion drive, making much better use of the slower 2TB drive.
I measured the drive speed and it was over 2000MB/s for both read and write. Note: if you are getting separate enclosures and NVMe drive, you need to check each of them for what speed they are rated for. See below for tech details.
There was a good discussion on MacRumors that discusses many aspects of this exact upgrade including more of the technical details of different options.
*I’ve used/still own 4-5 different non-SSD Silicon-Power Armor A80 and A85 drives including living/traveling throughout Asia with a lot of time on dive boats and never had a problem with any of them. Great drives.
Tech Deep(er) Dive:
When researching NVMe enclosures, there are a few things to be aware of. As mentioned in the article, they are PCI based, so just like USB, there are different versions/speeds and configurations possible that affect performance. The controller used for NVMe enclosures/drives makes a huge difference. Just because it’s thunderbolt 3, doesn’t mean it will be that fast. You need to also check the number of PCI lanes a controller uses, or at a minimum what speed the enclosure supports as well as the drive itself.
As an example, I was considering getting the OWC Express 4M2 enclosure, which is a 4 slot NVMe enclosure. I was thinking I’d start with one or two drives, could mirror them, and expand as needed in the future. But what took a lot of digging to find out, each slot in the enclosure only uses 1 PCI lane, limiting each to ~750MB/s. To get the full listed performance of 2800MB/s you’d have to fill all four slots and stripe a volume across all of them. Putting a fast 2500MB/s NVMe drive into that enclosure would be a waste, since each drive would be limited to 750MB/s, and if only using 2 drives, the speed would also be limited to ~1500MB/s. To be fair, that’s probably also why it was half the price of a single drive enclosure rated for 3000MB/s.
There was a review of the TEKQ and similar drive that I found useful. It’s also a good historical reference for how much prices have dropped.