Does the 2019 iMac use proprietary SSD controllers or connectors?

Solution 1:

These models support a 7 mm tall SATA III (6 Gb/s) hard drive for the primary storage (using the 3.5" size as opposed to the 2.5"), and getting the Fusion drive means you'll also get what's called a small "blade" SSD via a PCIe connector. Without this you can't attempt what you're wanting to do.

So, in summary:

• Primary storage uses a 3.5" 6 Gb/s SATA 3.0 connection
• Secondary storage uses a NVMe PCIe "Blade" connection

However, getting a compatible "Blade" SSD is not as easy as walking into any computer store and buying one. You will need to purchase one from a vendor that specialises in Mac hardware to ensure you get what you need.

I haven't actually seen inside a 2019 model 27" iMac yet, so can't give you a definitive answer in terms of height clearance.

Recommendation

Before biting the bullet and ordering one of these machines, I would take a look at the following to ensure you can get what you need and also do what you want.

• How to Upgrade or Install a SATA Drive (SSD or HDD) in the Main Drive Bay of a 27-inch Apple iMac (2012 - Current)
• How to Upgrade or Install a PCIe SSD in a 27-inch Apple iMac with Retina 5K Display (2019)
• OWC Solid State Drives For iMac 27-Inch (Late 2013 - Current)
• OWC Complete Hard Drive Upgrade Kit Including Tools for all 27" iMacs 2012 & Later

Since you have the skills necessary to perform this upgrade, I would focus on ensuring you can actually get a compatible SSD with the storage you need.

NOTES:

1. For something like this I also recommend contacting OWC to discuss your requirements.
2. There would be no need for you to get the 3TB capacity Fusion drive in order to be able to do this, unless you want that capacity for whatever you intend to do with it later.

^{Disclaimer: I have no affiliation whatsoever with OWC.}

Solution 2:

Yes, Apple is still using proprietary SSD connectors as of 2019.

I purchased the 27-inch 2019 iMac with the Fusion Drive option and it contains the following for internal storage connections on the logic board:

• 1x proprietary Apple SSD "blade" connector.
• 1x proprietary SATA power connector.
• 1x industry standard SATA III data connector.

To upgrade the SSD:
I needed to purchase an adapter to connect the 2TB Samsung EVO Plus I ordered. The adapter I received had no electronics on the board whatsoever, so apparently Apple's proprietary connector is just a standard NVMe connector with different pin layout (although it might have other functions in other models).

NOTE: The SSD connector's position on the logic board only gives 5mm of clearance with the iMac's tapered aluminum body! If you're going to use a heat sink on your SSD (which is highly recommended for the SSD I bought -- it runs HOT), you MUST use a low-profile heat sink. The first one I ordered was 7mm tall and it didn't fit.

To upgrade the HDD:
I was able to use an ordinary 3.5" 16TB SATA III desktop hard drive without issues. The drive bay is nearer to the center of the iMac's body so it has much more clearance. The drive I ordered was 12mm high (which is pretty thick), but it fit just fine with room to spare.

NOTE: Even though the SATA data header on the logic board is standard, the power header is not. Connecting a standard SATA drive requires the use of a proprietary cable for getting power to it. The Fusion Drive models obviously include the cable, but I don't know if non-Fusion Drive models come with one.

I am happy to report that after putting everything back together, it runs like a champ. I've put this machine through some stress testing over the past couple months and have encountered no issues whatsoever. Fans run at normal speeds, and the SSD temps barely go above 50°C.