Windows 10: Slow downs in SATA transfer speed

I am having an odd issue where the transfer speed from my HDDs slows to unusable after my computer has been on for a while.

I have my primary OS drive as a Samsung 850 1TB SSD, paired with 2 x 3TB Toshiba HDDs. When I first start my computer, everything works as it should. After a few hours however, the transfer speed from the 3TB drives slows to around 200kb/s (according to resource monitor). At this speed the drives are almost unuseable. I am not quite sure where to begin diagnosing this problem. I am using an Asus Maximums VIII Hero Alpha motherboard with Core i5 6600K Skylake processor, and 16GB. As far as I can tell the problem is limited to the HDDs, not the SSD. Looking at resource monitor, there is nothing lse using the drive that would explain the transfer speed, and there are no processes overusing the CPU. There are no programs installed on the HDDs, they are used for storage only. One stores my docs, music and video files, the other contains backups of the other drive created by EaseUS. I use Intel Rapid Storage Technology drivers.

If any other information would be useful, please ask. Any assistance would be appreciated.

:)

 

Copy speed very slow in windows 10

Using NAS Network mirror drive SATA to NEW USB3 thumb drive.

If downloading from the 120 MB Internet to the NAS, speed is constant.

PC SATA to NAS, speed is high.

The USB 3 starts out high, then drops off.

You might look at:

Speed up and fix slow USB flash pen drive data transfers

Windows 10 may have added this API (see step 3 here)

Windows 10 Slow Deleting or Moving Files? Here’s the Real Fix

 

making a USB hard drive my primary hard drive

The internal SATA connection is significantly faster than USB read/write speeds. I'm no expert so I can't say how this would slow down the OS.

 

Change the SATA slot may helps.

 

Did try that, same issue. Will try using the ASMedia controller though to see if it's and Intel issue. Have also updated my motherboard BIOS to the latest. At my wits end with this, brand new computer, brand new drives, and they are unuseable. Temps on the HDDs are around 35C, so not overheating. AHCI is enabled in BIOS. Drives are reporting as healthy in any and all analysis applications I've used.

Any other suggestions? Never come across this before so don't know what to do.

 

Tried putting the drives on the ASMedia controller but the same issue seems to occur after a day or so, albeit not as severely as on the Intel controller. Still only getting around 4MB/s transfer speeds. Could it have anything to do with the computer going to sleep and the drives not waking properly?