Identical drives have same write speed but different utilization in Windows stripe

[FreemoX]

System specs:

Spoiler

OS: Windows 10 x64
CPU: AMD Ryzen 5 2600x
RAM: 4x16GB (64GB) DDR4
PSU: Corsair RM750x
GPU: GTX 1060 6GB
MoBo: ASUS ROG STRIX B450-F

Drives in question: WDC WD30EFRX

I've ran my own NAS and NC-server for the past two years now, which is a stock HPE Microserver Gen8 4GB running Ubuntu 18.04.
I've always had performance issues with it despite services being optimized for the 4GB RAM, and later found it was related to the HDDs (Main storage was RAID10 composed of two identical 3TB and two 8TB WD Red drives, where each stripe consisted of one 3TB and one 8TB drive), but didn't give it much more thought after all drives passing my health checks.

I am now in the process of swapping the two 3TB drives for two new 8TB WD Red Pro-NAS drives. I've done the proper backup procedure with both local and off-site (of course), and I have now placed my two 3TB drives into my desktop computer and striped them using the Windows Disk Management tool. These will be used for local storage, but their current purpose is to hold the data from the server RAID until I've done a fresh setup on the server.

Now, during the transfer phase from one of the new 8TB drives to my striped 3TB drives, I noticed that one of the 3TB drives are consistently running at ~92% utilization while the other 3TB drive hits at a comfortable ~30% utilization, despite both drives having the same write speed during the transfer.

Now again, all three pairs of HDDs are identical; two (old) 8TB drives (WD80EFAX), two (old) 3TB drives (WD30EFRX), and my two new replacement 8TB drives (WD8003FFBX).

The screenshot of the task manager summary shows the new 8TB WD8003FFBX (which is being copied from, after having the data moved from the server to it) as Disk 1 ( F: ), and the two older 3TB WD30EFRX striped drives as Disk 2 & Disk 3 ( G: )

I'm not as experienced with RAID, I've only used the Windows built-in tool, and Linux's mdadm, but I think it would be reasonable to expect two identical striped drives to have the same utilization during the same task.. Any ideas, tips, or explanations would be greatly appreciated.

TLDR;
Two identical Western Digital drives (WD30EFRX) perform differently in a striped Windows 10 environment.

[Bombastinator]

The headline seems to be missing an important “not” or some such and therefore reads a bit like “water is wet” might mean the thing is simply not getting read. 
 

im not at all sure if those is the issue, but the 8tb drives are visibly not identical as they have different part numbers.  There could be vast internal differences other than size.

[FreemoX]

3 hours ago, Bombastinator said:

The headline seems to be missing an important “not” or some such and therefore reads a bit like “water is wet” might mean the thing is simply not getting read. 
 

im not at all sure if those is the issue, but the 8tb drives are visibly not identical as they have different part numbers.  There could be vast internal differences other than size.

Yeah, not quite sure how to title my issue differently.

But yes, the 8TB drive pairs are different because no local retailers sell my old drives anymore, which is the reason for the newer model.
Although the issue resides in the 3TB drives, not the 8TB ones