I’m experiencing unusual behavior with my system. Recently, I purchased this item from Aliexpress
https://www.aliexpress.com/item/1005005653706057.html
You can see here a picture https://ibb.co/5jMd4p2
Is a M.2 to Sata3.0 Adapter Card 6 Port III 6GB/S Pci-E Controller Card equipped with the ASM1166 chip.

The system where it is installed is a MiniPC with an Intel N100 that has two M.2 slots. In one of these slots, there is an NVMe drive that contains the operating system (Ubuntu 24.04). The other slot is occupied by an adapter that controls four separate SATA HDDs, each with a capacity of 3TB. I prefer to use them individually rather than in RAID. Then i have 4 disks (/dev/sda /dev/sdb /dev/sdc /dev/sdd) mounted on 4 directories (/drv/diska /drv/diskb /drv/diskc /drv/diskd)

I conducted a system test using the command dd if=/dev/random of=/drv/disk[DiskLetter]/rand.txt bs=1M count=100. This command writes a random file to the [DiskLetter] disk. I executed this command simultaneously on all disks during the test.

Everything operates smoothly as I write to three disks simultaneously. In this setup, each disk maintains a writing speed of approximately 150MB/s. Consequently, I can achieve a combined writing speed of 450MB/s across the disks. I have the flexibility to choose any configuration I prefer. For instance, whether I write to sda, sdb, sdd, or sda, sdc, sdd, or even sda, sdb, sdc, the speed remains consistently around 450MB/s.
Here you can see a screenshot of nmon when I write on three disks simultaneously https://ibb.co/h18NQJ2, here when I write only on one disk https://ibb.co/KFSG4bB

The issue arises when I attempt to write to four disks concurrently. In fact, the last disk (/dev/sdd) consistently operates at a speed ranging from 40MB/s to 5MB/s. The other disks perform well, maintaining a speed of about 150MB/s each.
Here you can see a screenshot of nmon. https://ibb.co/Hhck7SS

I expected that the workload across the disks would be more distributed, with each disk achieving a speed of 100MB/s. However, it appears that the system prioritizes the first three disks in sequence, causing the fourth disk to suffer from insufficient bandwidth.

I’ve tried everything, including physically rearranging the disk order on the adapter and using different disks. However, the last disk (sdd) consistently shows a speed of 40MB/s to 5MB/s, while the others maintain a speed of around 150MB/s.

It appears that the I/O scheduler in the operating system is not balancing the data transmission effectively. It could also be an issue with the controller. I’m uncertain and would appreciate assistance in resolving this mystery.

Famous last words before problems ensue. That being said, I think it is probably a hardware issue. Three drives running full throttle through that M.2 slot may be saturating the available pcie lanes. I believe the N100 has a max of 9 pcie lanes compared to the typical intel Core cpus with usually 20. The motherboard that the cpu is on will also impact the number of pcie lanes available and what's allotted to that M.2 slot. Don't know what's on your N100 motherboard. Also, the way your adapter's firmware meters out the available bandwith may be coming into play as well.

The problem may be that /dev/random is overloaded. Reference the Wikipedia article: https://en.wikipedia.org/wiki//dev/random

/dev/random is seeded with entropy from environmental noise, collected from device drivers and other sources. If not enough entropy is available then /dev/random is blocked until enough entropy is built up to appear to generate random numbers. Thus if you overload /dev/random it may periodically block itself for a while. Maybe you can run three /dev/random threads simultaneously without blockage and get blockage when you try to run four threads simultaneously.

You could test out this idea by getting your source data from /dev/zero instead of /dev/random.

Your contraption is a port multiplier https://www.asmedia.com.tw/product/4...dYQ8bxZ4UR9wG5, and not just any port multiplier but a cascaded port multiplier - it is as bad and as cheap as it can get. It connects multiple daisy-chained devices to a single data lane and, when bandwidth is saturated performance suffers. How exactly it suffers is determined by the multiplier, you can do nothing about it on the OS level. Connecting disks to different ports might help though.

Are there any alternative products, possibly at a higher price point, that I can purchase?

I require an M.2 NVMe to SATA converter with 6 ports
.

I don't think so. You want to add several disks to you PC, there are different ways of doing it and plugging them all into a port designed for just one disk is not a good solution. A nice PCIe SATA card with proper independent ports is the right way to do it. But if you are bent on using port multipliers (or just don't have free PCI slots), look for those that support FIS.