I bought the iMac with 8GB of RAM, 2x4GB SO-DIMMs.

I wanted to upgrade to 128GB - 4 sticks of 32GB. I replaced the 8GB with 128GB (4x32GB). Same brand OWC, same power etc.

If I only use 2x32GB, in slot 1 & 3 or 2 & 4 it works. I tried 2 sticks and the 2 others; all 4 work. But, if I use all 4 at the same time, I have a black screen.

Any tip please?

  • Which iMac specifically are you attempting to upgrade and what are the memory specs?
    – Allan
    Oct 27, 2020 at 15:25
  • What happens if you try to run Apple Hardware Test (does it load? errors?)?
    – Joy Jin
    Oct 28, 2020 at 1:25

2 Answers 2


The 2019 iMac computer have different memory specifications depending on which model you get - the 21" and 27"; the 4K or 5K models. Not all models support 128GB of RAM.

It's also important to note that OWC is not the original memory. OWC is an after market brand. If you're positive all of your memory works, but when you install the rest you get a black screen, you may be trying to install too much. Confirm you have the right memory for your particular application.

The 21" models officially support up to 32GB of RAM and the 27" models 64GB. However, independent testing per EveryMac.com confirms that they can support 64GB and 128GB of RAM for the 21" and 27" models respectively.

You should be installing these in pairs, but there's no requirement to stagger the installation (slots 1 & 3 or 2 & 4). See Install Memory in an iMac. Memory is usually sold in "kits." A 32GB kit is two 16GB SO-DIMMs and a 64GB kit is two 32GB SO-DIMMs. So, for example, for the 21" iMac, you'll need two 32GB kits which will have four 16GB SO-DIMMS for a total of 64GB of RAM.

Here's a breakdown of the maximum memory per slot for each model:

  • 21" iMac → Max RAM per slot = 16GB
  • 27" iMac → Max RAM per slot = 32GB

Check the serial number on the ram. Install the ram in order of serial numbers. It may be that they have to be matched. This worked for me on my Hackintosh. Your iMac performs a memory initialization procedure when you first turn it on after upgrading memory or rearranging DIMMs. This process can take 30 seconds or more, and the display of your iMac remains dark until it's finished. Make sure to let the memory initialization complete.

  • The motherboard and more importantly, the CPU requires this, not the memory itself. Macs don't have this requirement. Memory isn't serialized and matched like CPUs have to be.
    – Allan
    Oct 29, 2020 at 17:09

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