Adding RAM

I disagree!

Minimising the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) means that the capacity in GB of the memory modules to be installed in Channel A should match as closely as possible the capacity in GB of the memory modules to be installed in Channel B! If you would put the 16GB modules in slot 1 and 2 or in slot 3 and 4, you would actually do the opposite of what is recommended, i.e. you would be increasing the capacity difference between Channel A and B: 32GB in one channel versus 64GB in the other one...

In other words and in this case, and looking at the image in the reply above:

1. put one 32GB memory module in slot 1;
2. put one 16GB memory module in slot 2;
3. put the second 32GB memory module in slot 3;
4. put the second 16GB memory module in slot 4.

Because slots 1 + 2 make up Channel A and slots 3 + 4 Channel B, in this case both Channels have the same amount of RAM at their disposal (in this case, each channel has 48GB), which is exactly what Apple recommends by saying "minimise the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4). Also, I recommend putting the highest capacity memory modules (in this case the 32GB modules) in slots 1 and 3.

After installation of the memory modules, you can check if you did the right thing in the "Hardware > Memory" section of System Information (you find that application inside the Utilities folder on your iMac). I add a screen capture of my memory banks's layout to help you understand better (look at "Channel A" and "Channel B": the same amount of memory). Also know that Bank 0 equals Slot 1, Bank 1 = Slot 2, etc. In my case each Channel has 20GB at its disposal.

According to Apple: "iMac (Retina 5K, 27-inch, 2020) DIMMs have a notch on the bottom, slightly left of the middle. If your DIMMs are mixed in capacity, minimize the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when possible." From: Install memory in an iMac - Apple Support

You should install two like capacities in slots 1 & 2 and the other two in 3 & 4. It doesn't matter which two go in which slots. So the two 16GB modules can go in either slots 1 & 2 or 3 & 4.

Yes, I think that's the best solution: run your iMac with 'only' the two 32GB 2666 MHz modules installed, in slots 1 and 3 (see the diagram that dialabrain posted above). With 64GB of RAM your 2020 iMac will be able to serve you very well.

No need to thank me. Helping others helps me to expand my knowledge too. I live in Europe and can't really come over for a beer or coffee, but appreciate the offer much! :-)

Let us know, if you want, if everything runs fine and at 2666MHz with the 2 x 32GB installed.

Dual Channel functionality? I will explain it in a simple way. Imagine your iMac as being a car with a big engine and lots of power. It will run better and faster if you allow it to use a lot of fuel, which we could compare to the amount of RAM you install. Instead of just installing a fuel tank of a 'normal' size (let's say your current 32GB of RAM), it's possible to install one that is twice as large (your new 64GB of RAM). That's already nice. However, in a non-Dual Channel world, all the 'fuel' needs to find its way to the engine by running through only one tube, one 'channel'. In your Dual Channel car/iMac, there are actually two fuel tanks installed, each with its own 'channel' through which the fuel reaches the engine (the processor). Result: the same amount of 'fuel' (the RAM or Random Access Memory that is 'consumed' or used by the operating system and the applications running) reaches your engine twice as fast in this 'Dual Channel' system as it would in a non-Dual Channel system.

This is also why Apple recommends, in case you install memory modules of different sizes, to minimise the difference in capacity between Channel A (slots 1 and 2) and Channel B (slots 3 and 4). In other words, it is best to have the two 'fuel tanks' as identical in size as possible.

I hope this answers your question about Dual Channel technology. It is a matter of allowing the Intel processor to access the RAM much faster.

I'm pretty sure that if you "only" install the two 32GB modules, one in slot 1 and one in slot 3, your iMac will be more than happy to run your applications very smoothly. That way you avoid having either the memory modules' speed lowered or the Dual Channel functionality disabled.

I've done some research and you're not the only one who has this issue. Because you seem to be stressed out with this, I will simply give you the answer to the problem. Don't bother about checking out if your new 32GB memory modules are indeed 2666MHz or not, because they most certainly are! It turns out that 2020 iMacs are more picky about which memory modules they like and which they don't really like so much.

The problem is that if you don't have four totally identical 2666 MHz memory modules (same make, and identical specs, etc.), your iMac will use them at 2133MHz.

Now, you have three options:

1. Keep your current setup, and have your iMac use its memory at a lower RAM speed (2133 instead of 2666); the advantage being that Dual Channel will still be functioning.
2. Put the 16GB modules in slots 1-2 or 3-4 and put the 32GB ones in slots 3-4 or 1-2 respectively. Advantage: your iMac will use its memory at full speed. Disadvantage: you lose Dual Channel functionality.
3. Only install the two new 32GB memory modules, in slot 1 and in slot 3, and forget about the 16GB ones. Advantage: your iMac will use its memory at full speed and will keep its Dual Channel functionality. Disadvantage: you will 'only' have 64GB of RAM instead of 96GB.

I totally recommend the last option, as in my experience 64GB of RAM should be more than enough to have even more demanding applications (e.g. photography and video editing software) run smoothly. The disadvantages of either losing Dual Channel functionality or having lower memory speeds are, in my opinion, more serious than having 64GB RAM instead of 96GB.

I'm far from sure that they are 2666MHz, actually. When memory modules of different speeds are installed together, an iMac will run all of them at the speed of the slowest module. Obviously, your iMac needs all RAM modules to be running at 2666MHz and therefore their specs need to be the same.

Presuming your two 16GB modules are the original modules that were installed in your iMac when you bought it (first hand?), they are not to be suspected as they will run at 2666MHz. I suspect one or both of the newly purchased 32GB modules are 2133MHz instead of 2666.

Please answer the following questions:

1. Where did you buy the two 32GB modules?
2. What make are they?
3. Can you make a photo of the label on the plastic packaging of the modules and post it here, so we can see the full specs?
4. Can you please run "System Information", go to the Hardware section, and click on "Memory" in the left pane. Then try to make a screen capture of the right pane, where you will see the overview of all memory banks and what is installed in them, and post it here.

It is always possible that the two 32GB are defective. I have no first-hand experience with OWC memory modules installed in iMacs. I always use Samsung and never had an issue, not even in combination with the modules installed by Apple. For now, all I can say is that many people using a 2020 iMac and installing third-party memory modules, have problems like yours.

It looks like you have bought these: https://bermuda.desertcart.com/products/127113825-owc-32-0-gb-2666-m-hz-ddr-4-pc-4-21300-so-dimm-260-pin-memory-upgrade-owc-2666-ddr-4-s-32-g-for-2018-mac-mini-macmini-18-1-2019-27-inch-i-mac-i-mac-19-1-and-pc-laptops

If they are, they should be totally compatible. One user commenting on that page, says he had to install the two modules in slots 2 and 4 (and not in 1 and 3) to make things work...

I don't think it has anything to do with the order you installed the modules in. Although I recommend always putting the largest modules in slot 1 and 3, the opposite order works as well. I suspect the cause of the problem lies in the fact that at least one of your modules is 2133MHz and not 2666. It would be helpful to see the labels on the packaging, although that is not a guarantee that the modules actually have those exact specs. The labels on the modules themselves should tell us the truth, unless someone fiddled with those as well. But first things first: let's see those labels on the packaging. I'm on the Old Continent, so will probably not be able to reply in the next 12 hours or so.

In the mean time, don't use that iMac and turn it off completely, as it's not supposed to run its RAM at lower speeds than 2666MHz. It could be OK but better be safe than sorry.

Change the orientation, test the 16GB modules in the 1 2 or 3 4 slots and the 32GB modules in the 1 2 or 3 4 slots.

If that doesn’t help, then there is something wrong with those 32GB modules and you need to return them and buy Mac compatible RAM directly from Crucial or OWC.

Are you sure you installed the modules correctly, i.e. did you feel/hear a click? Try uninstalling and re-installing them. What make are they?

These are the necessary specs:

For optimal memory performance, DIMMs should be the same capacity, speed, and vendor. Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• • PC4-21333
  • Unbuffered
  • Nonparity
  • 260-pin
  • 2666MHz DDR4 SDRAM

FWIW, I believe Deep Sky Diver is correct despite my Apple Recommended. However, I don't see any recommendation as to whether the smaller or larger modules should go in slots 1 & 3 vs 2 & 4. Personally I have always avoided having anything but 4 identical modules.