Chosen Solution
Hello, 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.
UPDATED ANSWER 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: https://allinfo.space/2021/08/17/crucial… 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!
“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.
