iMac Retina 5K, 27-inch, 2017 won't boot after RAM upgrade to 64GB

[wolvesfc]

Currently my iMac Retina 5K, 27-inch, 2017 has 4 x 8GB DDR4 (2400MHZ) OEM RAM installed.

I wanted to max out the RAM to the full 64GB allowed for my model, so I purchased 4 x 16GB DDR4 RAM (2400MHz) to replace the OEM RAM from the following link on Amazon.

https://www.amazon.co.uk/gp/product/B07YDR5MXK/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

But when I replace the OEM RAM with the 4 newly purchased RAM, my iMac won’t boot up (no Apple logo or chime sound) just stuck on a black screen when the power button is pressed. I've left it in this state for a few minutes as I understand it takes 30secs or so for the iMac to recognise the new RAM, but still nothing.

If I remove all the OEM RAM and install 1 of the newly purchased RAM sticks on its own, the machine boots up. I’ve tested all 4 of the new RAM sticks individually and all 4 boot up the machine and are recognised.

I can then add a 2nd and 3rd newly purchased RAM stick and the iMac still boots up. But once I add the 4th new RAM stick the iMac fails to boot and remains on a black screen.

If I then remove the 4th stick and put back 1 of the OEM sticks along side the 3 other newly purchased sticks the iMac boots up.

Is there a reason I can’t get my machine to power up using all 4 newly purchased RAM sticks? Or is there something I’m missing and need to do? Any help or suggestions would be welcome. Its as if any combination will work as long as its not trying to use the full 64GB size.

Combinations I have tried are:

1-New 16GB (Tried with all 4 sticks)
2-Empty
3-Empty
4-Empty
iMac Boots up

1-Empty
2-New 16GB
3-Empty
4-New 16GB
iMac Boots up

1-New 16GB
2-New 16GB
3-New 16GB
4-Empty
iMac Boots up

1-New 16GB
2-New 16GB
3-New 16GB
4-New 16GB
iMac FAILS to Boot up

1-New 16GB
2-New 16GB
3- Old 8GB
4- New 16GB
iMac Boots up

 

[wilberforce]

I don't think you are missing anything, and you seem to have tried all the combos I would have tried.
Not sure what I would do in your position, maybe:
1. Keep trying
2. Try another brand, like Crucial
3. Put in 2x16 plus 2x8 sticks and settle for 48GB

 

[wolvesfc]

I don't think you are missing anything, and you seem to have tried all the combos I would have tried.
Not sure what I would do in your position, maybe:
1. Keep trying
2. Try another brand, like Crucial
3. Put in 2x16 plus 2x8 sticks and settle for 48GB

Thanks for the reply, yeah, I find it strange that all the sticks work when inserted individually or in pairs or in groups of 3. But only fail to work when all 4 are inserted together.

I've accepted I probably won’t be able to get the full 64GB using these sticks and currently I’ve got 3 of the new sticks (16GB) inserted along with one of the original OEM sticks (8GB) giving me 56GB.

Would you recommend I go down to 48GB (2x16GB and 2x8GB) to have the same sizes in the channels or is the current 56GB okay?

 

[wilberforce]

Would you recommend I go down to 48GB (2x16GB and 2x8GB) to have the same sizes in the channels or is the current 56GB okay?

If you have 48GB, each channel has the same amount of RAM, and will thus run in full dual channel mode.

If you have 56GB, i.e., 24GB in one channel and 32GB in the other channel, then it will run in flex mode, which means that 48GB of the 56GB will run in dual channel mode, and the remaining 8GB will run in single channel mode (half the effective transfer speed).

If you are not typically using all of the 56GB RAM, it would be better to put in just 48GB RAM, so all your RAM runs at full speed.
If you are typically needing all the 56GB RAM, it would be better to put 56GB RAM, because the 8GB RAM that is running at half the effective speed is still much faster than swapping memory to disk/SSD.

You can do some memory transfer speed tests using NovaBench to compare, but be aware that you have to run it at least ten times to get meaningful results, as there is a lot a variability from run to run for various reasons (background processes, for example).

 

[Fishrrman]

OP wrote:
"I've accepted I probably won’t be able to get the full 64GB using these sticks and currently I’ve got 3 of the new sticks (16GB) inserted along with one of the original OEM sticks (8GB) giving me 56GB."

Fishrrman's Rule #3 of Macs:
==================
When one pathway doesn't work for you,
But another pathway does,
then...
Use the path that works.

 

[maverick100]

Hi, I go the ram for my 2017 from OWC; not sure if there is a difference in price from what you paid.
It is was installed in my iMac one week after my iMac arrived. Not one problem memory problem since it was installed.

 

[wilberforce]

Hi, I go the ram for my 2017 from OWC; not sure if there is a difference in price from what you paid.
It is was installed in my iMac one week after my iMac arrived. Not one problem memory problem since it was installed.

Yes, I suggested OP try another brand (OWC and Crucial are good choices), if returning is an option (OP appears to be in UK)

 

[lasloduncan]

I have the same problem nov 19.....i really really dislike debuffing unnessary crap like this......makes no sense also form owc 194.00 not worth anything if they dont work