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I have replaced built-in SSD and HDD with: Samsung 970 Evo Plus 2TB (got the latest firmware version)Samsung 860 Evo 2TB At the first boot, in the recovery menu (alt+cmd+r) I have formatted both of these drive to APFS and merged them into one partition (diskutil resetfusion command). After installing Big Sur everything was fine - benchmarks reported ~3000MB/s write/read speeds, OS reported 4TB of free space in a single partition. After a while (and filling my disks with 3.5TB of data) my write speeds are ridiculously slow - Blackmagic Disk Speed Test shows something between 90-300MB/s, and from time to time it even drops to zero and system freezes. Also, the read speeds are extremely inconsistent - one time it is 900MB/s, the other 1800MB/s (but usually around 1800MB/s). I am wondering if this might be an overheat problem - while doing benchmarks I am getting: Temperature Sensor 1: 58 CelsiusTemperature Sensor 2: 78 Celsius And Samsung SSD Temps: 950, 960, 970, 750, 840, 850 and 860 EVO and PRO series are rated for operation between 0ºC and 70ºC. In idle, when I am not doing anything the temps are not much better: Temperature Sensor 1: 51 CelsiusTemperature Sensor 2: 66 Celsius I also replaced CPU to i7-7700, the temps in idle are fine: Fan: 1207 rpm CPU die temperature: 48.55 CGPU die temperature: 38.00 C The first round of benchmarks gives better results (500MB/s / 2800MB/s), but everything after is limited - clearly a throttling does occur. Have you encountered such issue? I can install some thermal pads and a radiator on M.2 SSD, but still - this does not seem normal. Can a cheap-ass adapter from Aliexpress be the reason? Edit: In MacRumors: internal SSD is overheating, how to fix thread I found the following information: P.S.: Also never fill up your SSD to more than 85% to 90% of its capacity. Mac OS creates lots of temporary files (logs, swap, etc.). Once only 10% to 15% of available disk space remain, the amount of disk I/O seems to pick up noticeable. And someone suggested to do sudo trimforce enable Which I will try Update (02/09/2021) I’m not sure I follow why you created a Fusion Drive using these two SSD! Samsung 970 EVO Plus 2TB M.2 PCIe/NVMe SSD & Samsung 860 EVO 2TB 2.5” SATA SSD 2 reasons: I prefer to have one partition over manyWhen I will be selling this Mac it is better to have it this way - an average Joe is too dump to grasp a concept of partitions Basically, the PCIe SSD is not seen as a discreet drive within the macOS (look at your disk information pane under The About This Mac Storage tab all you’ll see is the one Fusion drive of the HDD’s space of 2 TB (the 2 TB of the SSD in your case is hidden!) Incorrect. I see 4TB Fusion Drive. Also, diskutil list command shows all 3 drives (2 SSDs and Fusion Drive).

I guess I will try to split Fusion Drive into two separate disks, but this still does not explain the temps I am getting. In the previous iMac I had Apple’s NVMe SSD + Samsung 850 Evo and my temps where completely fine (and I also had Fusion Drive).

I’m not sure I follow why you created a Fusion Drive using these two SSD! Samsung 970 EVO Plus 2TB M.2 PCIe/NVMe SSD & Samsung 860 EVO 2TB 2.5” SATA SSD I would never setup a dual SSD Fusion Drive. That defeats the purpose of why the Fusion Drive was created in the first place which was to off set the cost of the more expensive SSD with a cheaper HDD drive. Gaining the performance the SSD without the heavy cost of a much larger SSD. Remember the function of the SSD in a Fusion Drive set is acting as a Cache for the HDD! Basically, the PCIe SSD is not seen as a discreet drive within the macOS (look at your disk information pane under The About This Mac Storage tab all you’ll see is the one Fusion drive of the HDD’s space of 2 TB (the 2 TB of the SSD in your case is hidden!) So each write is written the the PCIe SSD then written to the SATA SSD which is why your system is so slow as the depth of the SATA I/O buffers are not deep enough Vs your PCIe/NVMe ability to write. So it will be slower as you discovered! So what is the ideal setup here? I would break the Fusion Drive set and just use the PCIe/NVMe as the boot drive hosting my OS and my apps with the rest of the drive unused! I mostly stick with 512 or 1TB drives for this depending on what the client workflow is. In some cases I have used larger PCIe/NVMe drives but thats when that was the only drive in the system. If a second drive is used it is strictly the media drive holding the work as an example video snippets or music scores. In some cases its an internal drive, in many cases its an external RAID drive. So… what is your workflow? video, music production, CAD/CAM or image artist?