27-inch iMac flash storage cannot be replaced or upgraded
While a 27-inch iMac with 4TB or 8TB of flash storage has that storage in a slot, the entire line does not support user storage upgrades.

iMac internal storage may be fixed at purchase
The latest update to the 27-inch iMac included the T2 chip, which handles encryption and security on the computer. Because the SSD is fixed to the logic board for hardware encryption, changing the logic board will erase all data on the drive, and changing the drive is impossible for the same reason.
The document obtained by MacRumors says that the 4TB and 8TB configurations have a flash storage expansion board connected to the logic board via a connector. The expansion board is not present in the 2TB and lower configurations, nor are there third-party modules compatible with it. It also appears that the SATA port internal to the 2018 iMac 5K for Fusion Drives has been omitted in the new model.
The non-upgradeability of the flash storage isn't a big surprise. Apple uses the T2 to secure similar storage on the MacBook Pro, the iMac Pro, and the Mac Pro. Upgrading the Mac Pro requires a firmware update process that has to be executed from a different Mac, which isn't available for any other model, despite using the same storage slots.
The 27-inch iMac is available for purchase (find the best 27-inch iMac prices in our Price Guide) and can be configured with up to 8TB of internal storage, 128GB of RAM, and AMD 5000 series graphics. Customers can also opt for nano-texture glass for an additional $500, similar to the Pro Display XDR.

iMac internal storage may be fixed at purchase
The latest update to the 27-inch iMac included the T2 chip, which handles encryption and security on the computer. Because the SSD is fixed to the logic board for hardware encryption, changing the logic board will erase all data on the drive, and changing the drive is impossible for the same reason.
The document obtained by MacRumors says that the 4TB and 8TB configurations have a flash storage expansion board connected to the logic board via a connector. The expansion board is not present in the 2TB and lower configurations, nor are there third-party modules compatible with it. It also appears that the SATA port internal to the 2018 iMac 5K for Fusion Drives has been omitted in the new model.
The non-upgradeability of the flash storage isn't a big surprise. Apple uses the T2 to secure similar storage on the MacBook Pro, the iMac Pro, and the Mac Pro. Upgrading the Mac Pro requires a firmware update process that has to be executed from a different Mac, which isn't available for any other model, despite using the same storage slots.
The 27-inch iMac is available for purchase (find the best 27-inch iMac prices in our Price Guide) and can be configured with up to 8TB of internal storage, 128GB of RAM, and AMD 5000 series graphics. Customers can also opt for nano-texture glass for an additional $500, similar to the Pro Display XDR.
Comments
I have two questions:
1) Does the T2 chip provide encryption for external drives or is all you have using APFS encryption? If the latter, does APFS encryption work on RAID 0? I know OWC uses SoftRAID for their RAID but there are SSD hardware RAIDs available.
2) Have your seen any AJA disk speed test results on any of the new iMacs yet? It will be interesting to see the results on each storage size. A two blade RAID using ThunderBlade is supposed to reach 3800MB/s (might be two full RAID 0 ThunderBlades running as RAID 0, OWC's documentation isn't clear).
As for me, I'm waiting for the Apple Silicon Macs to be released and I might go with a laptop with external monitor this time although I haven't really taken a laptop on travel for a long time. Have fun with your iMac!
What we buy today & drool over is not what will be here in 2-3 years, but it will work.
The T2 chip also has another dirty dark secret. You can brick your Mac as a result of the T2 chip. Let's say you are in a location with no internet access and the Mac with the T2 chip won't boot into macOS. So you are thinking of your regular troubleshooting technique to restore the Mac. Boot into recovery mode and erase the drive and re-install from your bootable USB flash drive with macOS and restore from backup, or if a company-owned Mac, just erase and re-image using a USB boot disk. Most people are unaware that Apple's default T2 chip setting is set to disable booting from any external device and restrict OS installations to only validated copies from Apple. So after you erase the internal drive, and try to boot from an external drive, you have just bricked the Mac because the T2 chip blocks booting from an external drive. Now that the drive is erased, there is no more recovery mode and you cannot change the T2 chip settings to allow booting from an external drive. The only way to restore the Mac is through Internet recovery mode. So if you don't have internet at the moment, the Mac is bricked. You need to boot into recovery mode before you erase the drive and change the T2 settings first by allowing booting from external devices, and disable all security for the OS so it will allow the USB flash drive to install macOS. It is a big hassle if you get stuck in that situation.
Exactly! No reason why Apple should charge such excessive prices for both RAM and SSD upgrades. Here is where it gets even more absurd...Apple charges different prices for the same exact SSD upgrade based on different models of Macs and different tiers. The same 1TB upgrade can vary in hundreds of dollars between different Mac models and different tiers (base model -> top model). Charge more for lower models and less for higher models...yet it is the same 1TB SSD upgrade. That makes no sense.
Unfortunately, Apple engineered it that way on purpose with the T2 chip. Yes it provides security and other features, but it also makes it so restrictive, it cannot be replaced or upgraded later. So after AppleCare expires...how much is that 8TB SSD going to be if it goes belly up?
I totally get your reasons for the 8TB option, as I would do the same, but not with an Intel Mac when they are out the door in Apple's mind. Sure you will likely get about 4-5 years of use out of that Mac, but in past Apple history, they did not support the OS for very long when they switched processors. When they switched from 680x0 to PowerPC, the 680x0 Macs only had support for System 7 and early versions of System 8 (7.5, 7.6, 8.0, and 8.1. The original Power Mac shipped with 7.1.2). Apple cut off 680x0 Macs with System 8.1, and System 8.5 and later were PowerPC only. With Intel, the last iMac G5 only had support for Mac OS X Tiger and Mac OS X Leopard (10.4 and 10.5), and they originally shipped with 10.4. So they only received one OS upgrade. The PowerPC Macs were cut off when Snow Leopard shipped (10.6). As Apple moves forward with Apple Silicon, they probably are not going to put that much effort into supporting the Intel Macs except for maybe one or two versions past Big Sur. Maybe you should have done the 2TB SSD upgrade for the Intel Mac, and then do an 8TB option for the Apple Silicon Mac which will have a much longer future. $2,400 is pretty steep.
it does not make sense replace entire motherboard for simple repair and give back either newer or older on board soldered SSD.
Laptop maybe space issue die to battery size but on iMac?
I wonder mother board has another SSD slot which end user can add more SSD or not.
Soon someone will open this up and verify the layout of new iMac mother board.
I guess this will be last update for Intel iMac. As long as end user can additional SSD on mother board will be great.
Other wise wait for Apple APU iMac next year or so..
I have two BarraCuda Fast SSD attached to the back of my LG 5K using Scotch Indoor Mounting Squares and configured as a RAID 1. When I plug-in my MacBook Pro using one Thunderbolt cable to the monitor I get charging, monitor and RAID. Only issue, is to remembering to unmount them before disconnecting the MacBook Pro.
BarraCuda Fast SSD - Up to 540MB/s
https://www.seagate.com/consumer/backup/barracuda-fast-ssd/
When the price of FireCuda Gaming SSD come down to earth, I plan to swap out the Barracuda SSDs to FireCuda SSDs. Just doesn't make sense now being that the limiting factor is the LG 5K hub they way I have it set up.
FireCuda Gaming SSD - Up to 2000MB/s
https://www.seagate.com/consumer/play/firecuda-gaming-ssd/
If you run Blackmagic on the on the internal SSD and compare it to what is a relatively good external SSD like the BarraCuda Fast SSD that I referenced previously, you will see the SSD internal trounces the external SSD. To get faster you need to step up to a Thunderbolt enclosure and matching SSD, which is what you would need to install internally or better. That level of SSD is not inexpensive.