However, the information I found on the upgrade was contradictory. In the absence of concrete examples, advice such as "You'll kill the machine and maybe yourself" or "Well, it worked in my 2017 [iMac]" was little more than wishful thinking.
That faith (and general storage knowledge) proved to be well-placed after purchasing the iMac (it was a bargain I couldn't pass up). As a caveat, the 21-inch iMac with the regular hard drive does not have a PCIe SSD slot on the board, so you're out of luck.
But there is a practical method you can learn about—NVMe via Thunderbolt. Please continue reading if your iMac came equipped with a Fusion Drive or an SSD. Please be aware that the warranty on your iMac will be void if you attempt this fix.
Additionally, you risk irreparably harming your iMac if you proceed with this method. You shouldn't, but accidents do happen even when you're careful. Disclaimers aside, here's how to replace the NVMe SSD in your iMac with a more affordable, quicker, and larger drive.
Finding an NVMe SSD for Upgrade
As I suggested, there has never been any debate that the NVMe SSD could be upgraded, merely if it could be done for a decent cost. A big concern is that Apple employs proprietary connectors. OWC's compatible SSDs on Newegg were 40 cents a gigabyte, and Apple's on Amazon was an incredible $1 a gigabyte!
Although that's a step forward, I couldn't have a conversation with them because they were from new companies. It wasn't easy, but I finally came across a company called Fledging that offers 2TB Feathery drives for roughly 25 cents per GB.
Additionally, an iMac may only use an M.2 2280 SSD with an adaptor costing less than $20 from Sinetech. The Samsung 970 EVO Plus, a popular and fast mid-range NVMe SSD, has reportedly been unfriendly to Macs. I had meant to test the 970 EVO Plus as an upgrade; however, while formatting it in an external PCIe box, it wrecked my other Mac. I accepted the hint.
Installation of NVMe SSD
What's good about the Fledging NVMe is that it's a plug-and-play gadget. If you don't select a plug-and-play NVMe, you need to produce an installation USB flash drive with a full copy of macOS High Sierra or later.
There is no compatibility for third-party NVMe drives in macOS versions previous to High Sierra. It would help if you had the installation disc because when you remove the main drive in a Mac, you lose the potential to repair from the internet until you've rebuilt macOS.
Because I'm confident in its compatibility with Boot Camp, I went with High Sierra. Boot Camp's problems with Mojave may be over, but I'm sticking with what I know. A 2.5-inch solid-state drive (SSD) swap could also be an option. I did.
One thing I will recommend is to be careful detaching the display and treat it with care. iFixit has a great roller (see above) for smudging the screen, which greatly eases the procedure. When working with glass, it's best to clear a big area and eliminate anything sharp or hard that could cause damage to the surface.
Pad below the iMac and prop it, so it's leaned back. After that, the whole transaction consists of gingerly unplugging a few cables, unlocking quite a few Torx screws (you'll need Torx drivers), then extracting the speakers and power supply (don't touch the naked solder joints on the back), and logic board. Users find the NVMe SSD on the back of the latter, held by one screw.
Which Computers, Which Drives
As to performance, every drive I've tested, save for one that was still three times as fast as SATA, offers incredibly rapid seek times (10X quicker than SATA). Large, multiple-gigabyte transfers are the only time the faster throughput of the more expensive storage comes into play.
The more expensive varieties are recommended if you frequently work with huge video files or write enormous volumes of data.
To learn more about specific SSDs, see the reviews at PCWorld. If you move out the hard drive during your update and find your fans whirling at full speed, there's a simple workaround from CrystalIdea called Macs Fan Control which allows you to change all Mac's fan settings without the need for a costly cable.
