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resting

macrumors member
Original poster
Aug 15, 2011
53
2
So I bought this ram: Crucial 8GB 260-Pin DDR4 SO-DIMM DDR4 2666 (PC4 21300) Laptop Memory Model CT8G4SFRA266

It's not stated as mac compatible, but since it'd already arrived, I went ahead to try it out.

1605530604906.png


It's inserted into slots 1 and 3. Set the original ram in 2 and 4.

However, the speed, dropped to 2133 from 2666.

I believe this arrangement is the optimal arrangement, so the problem here is because it's not mac compatible?
Cause they do have a range of mac compatiable rams https://www.crucial.com/products/memory/mac/memory-for-mac

Also, from apple's recommendation, it recommends PC4-21333. Mine's PC4 21300. What does that mean and is that why the speed dropped even though it's DDR4 2666?

I assume it's better to have 24GB RAM at 2133MHz than 16GB RAM at 2666MHz?
My memory used is already hitting 15GB. It's going to pass 16GB easily.
 

Fishrrman

macrumors Core
Feb 20, 2009
22,745
8,582
If you TAKE OUT the "factory RAM" -- and leave only the Crucial RAM in -- does the clock speed go back up...?
 

wilberforce

macrumors 65816
Aug 15, 2020
1,015
962
SF Bay Area
The problem is not whether the RAM is "Mac compatible" or not, and PC4-21300 is the same as PC4-21333. The problem is that the 2020 iMac does not run optimally with mixed RAM sizes.
It is debatable whether it's better to have 24GB RAM at 2133MHz than 16GB RAM at 2666MHz.

Your best solution is to get another 2x8GB sticks (or exchange the 2x8GB for 2x16GB) and toss the Apple RAM.

Several other threads like this:
 
Last edited:

Mastagra

macrumors newbie
Nov 16, 2020
2
0
I have a similar setup to the OP. I was going to post a separate thread about it, but I will ask my question here, as maybe its of interest to the OP also.

I bought 2x16G sticks of Crucial. My mac is working fine with those two sticks as long as I also have the two original 8G sticks installed (but of course with the reduced speed the OP mentions. My mas is the latest 2020 27inch)

I want to only use the 2x16 that I bought, so I get the full speed. However, no matter what slot configuration I use, my mac does not boot when I use only the new 2x16. It will only boot when I use the 2x8 sticks along with the 2x16G.

I'm not very tech savvy and I researched the heck out of upgrading my macs memory, but this is totally confusing me. Any ideas or do I have a fundamental misunderstanding of something?
 

wilberforce

macrumors 65816
Aug 15, 2020
1,015
962
SF Bay Area
I have a similar setup to the OP. I was going to post a separate thread about it, but I will ask my question here, as maybe its of interest to the OP also.

I bought 2x16G sticks of Crucial. My mac is working fine with those two sticks as long as I also have the two original 8G sticks installed (but of course with the reduced speed the OP mentions. My mas is the latest 2020 27inch)

I want to only use the 2x16 that I bought, so I get the full speed. However, no matter what slot configuration I use, my mac does not boot when I use only the new 2x16. It will only boot when I use the 2x8 sticks along with the 2x16G.

I'm not very tech savvy and I researched the heck out of upgrading my macs memory, but this is totally confusing me. Any ideas or do I have a fundamental misunderstanding of something?
Put the 2x16GB in slots 2 and 4, and leave slots 1 and 3 empty. (Where slot 1 is at top).
Make sure the RAM is fully seated. It is quite easy to not seat it properly.
When you turn on the machine, wait 1 minute. The machine will just remain black and seem completely unresponsive, as this first boot after switching RAM takes extra time.
Report back again if still does not work after several tries.
 
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Mastagra

macrumors newbie
Nov 16, 2020
2
0
Hi and thanks for the reply.

Tried it again a few times and had the same behaviour.

2x16 in slots 2 and 4 - wont boot (waited 5 minutes)
2x16 in slots 2 and 4, original 2x8 in slots 1 and 3 - boots with all memory detected and working within 30 seconds


no matter what slot configuration I use, it doesnt boot without the original memory installed.
 

wilberforce

macrumors 65816
Aug 15, 2020
1,015
962
SF Bay Area
Hi and thanks for the reply.

Tried it again a few times and had the same behaviour.

2x16 in slots 2 and 4 - wont boot (waited 5 minutes)
2x16 in slots 2 and 4, original 2x8 in slots 1 and 3 - boots with all memory detected and working within 30 seconds


no matter what slot configuration I use, it doesnt boot without the original memory installed.
That is very unusual, I cannot explain or help with that behavior, as I and others have had no problem using 2x16GB of various RAM manufacturers. Crucial RAM also has the best reputation.
I can only suggest try contact Crucial, who have good support and warranty, and request a replacement two sticks to try out
 

haddy

macrumors 6502
Nov 5, 2012
416
133
NZ
Hi and thanks for the reply.

Tried it again a few times and had the same behaviour.

2x16 in slots 2 and 4 - wont boot (waited 5 minutes)
2x16 in slots 2 and 4, original 2x8 in slots 1 and 3 - boots with all memory detected and working within 30 seconds


no matter what slot configuration I use, it doesnt boot without the original memory installed.
I have the same 2020 iMac ....... 2x16. Slots 1 and 3. That is, 32G total. RAM from OWC. Works just fine.
 
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