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Model A1419 / Late 2013 / 3.2 & 3.4 GHz Core i5 or 3.5 GHz Core i7 Processor, ID iMac14,2

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Slow Write Speed (2TB Crucial Bx500 SSD) No Matter What


I have a late 2013 27" that I replaced the failing HDD with a new 2TB Crucial Bx500 SSD.

Write speeds are an abysmal 20-40MB/s no mattter what.

I have also replaced the SATA cable AND enabled Trim.

I have tried also formatting the drive to exFat and then back to APFS with a clean install of Catalina, and then migrated the data again with migration assistant.

Originally, I thought the drive or cable was bad, so I replaced both again.

The issue persists. Write speeds hover from 20-40 MB/s.

Thank you for your consideration and help. I will be sure to respond as promptly as possible.

Répondu ! Afficher la réponse J'ai le même problème

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Nice profile Picture!


Thank you, Ben!


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The issue is with the SSD's QLC NAND flash technology. Basically the write speed is because they switched from TLC to QLC. Had nothing to do with my hardware. More info here:

The Black Magic Disk Speed Test was showing slow write speeds because of the cache or whatever being full. We were giving it too much data to write in too short amount of time by running the speed test over more than a couple times.

@danj Just wanted to let you know what I have found!

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"Bingo! Your OWC is acting as the cache drive which you pulled. Put the Apple drive back in and break the Fusion Drive set if you can. Some macOS’s don’t clear so you’ll need to backup and wipe the HDD to clear its linkage to the Apple drive.

You’ll want to setup the SSD as your boot drive to get the performance. Install the MacOS onto it and copy over your Apps to it as well." @danj said here.

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Thank you for responding. Can you help me understand how this would apply to me? I am using just a 2.5” SSD without a PCIe drive.


@kellywilkins89 - Are you sure you didn’t buy a Fusion Drive system?

At this point would check your systems hardware configuration to make sure. Go to About This Mac under the Apple icon menu, then click on the System Report button to then check the NVMExpress section. Do you see an entry?



Hi Dan! Thanks for helping.

It says “this device doesn’t contain any NVMExpress devices…”

Also, I've noticed that the issue persists even when booting to the same drive via an external USB to SATA adapter


@kellywilkins89 - Sorry, I forgot this system uses a mSATA blade drive, go into the SATA listing. Do you see two drives?

If not I think you need to run the onboard diagnostics. Using a USB keyboard restart your system and press the D key to enter, did you get a error?



The SATA/SATA Express tab only shows the 1 device. The 2.5" 2TB which I installed.

This previously had a fusion, but I did what I always do and I plugged in the new 2.5" ssd in via USB and did an internet restore with a brand new OS, and then I migrated the data over.

Once the data successfully migrated, I removed both fusion drives and installed the 1 2.5" SSD inside.

Also, the diagnostics showed no issues.

I also tried with a different 2.5" SSD (PNY 1TB) just to see if the brand of drive was the issue, and the problem still persisted.


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Boy Named Kelly sera éternellement reconnaissant.
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