Strange speeds when transferring data between 2 NVMEs

[Namastyler]

Hello everyone,
I have a Z170A Gaming pro Carbon on the last bios update, W10 x64, 750W of psu, i7-6700k, GTX1070,  with 2 ssd, one is a Samsung 960 pro mounted directly in my motherboard's m.2 port, and the second one is a 970 evo plus mounted with a Glotrends adaptor M.2-PCIe X4, both cooled with Sabrent coolers.

The first one is a bit older and used, the second one is brand new, both validated by Samsung Magician as originals, both trimmed.

Now, CrystalDiskMark says they are both fine, as SEQ1M the 960pro gets R3479-W2002 while the 970evoplus gets R3538-W3296.

The rest is mostly the same too.

So far, all expected and welcomed, also starting both benchmarks at the same time I get the same results.

 

Now, if I try to copy a single file of 41GB for example things get strange:

• from the 970evoplus to the 960pro I can see a 600MB/s stable transfer speed;

• from the 960pro to the 970evoplus I can see a 60MB/s stable transfer speed. Now I'm not expecting 3400MB/s, 600 is still quite good, but that 60 is not right.

Things done:

There is no change if I use Explorer, Teracopy or ChoEazyCopy, always 600-60.

HWiNFO64 says both interfaces are NVMe 4X 8.0GT/s, so there should not be a problem there (I hope).

Zipping and unzipping large files is also mostly the same in speed on both hardisks.

Antivirus is disabled, closing all programs does not have an impact.

Doing the same test in Safemode gives the same results.

NVME drivers freshly reinstalled, no changes.

Malwarebytes says I'm fine.
Swapped pcie slots, nope.
Taken out a pcie-usb3.2 card, still nope.
 

If you have any idea to why or what should I try next to troubleshoot it please let me know, I'll update this post with every additional try.

EDIT\SOLUTION: uncheck the "compress this drive to save disk space" under properties of the disk.

[Mo5]

have you tried it in different pci-e slots? I feel like there's something that goes wrong when data goes between the cpu pci-e lanes and chipset pci-e lanes but somehow only in one way. could be some bios setting that affects these things too but i can't think of anything on the top of my head.

[Namastyler]

8 minutes ago, Mo5 said:

have you tried it in different pci-e slots? I feel like there's something that goes wrong when data goes between the cpu pci-e lanes and chipset pci-e lanes but somehow only in one way. could be some bios setting that affects these things too but i can't think of anything on the top of my head.

Thank you, yes, already done that too, no change.
I've even taken out the pcie-usb3.2 under it but there is still no change.
 

[Mo5]

1 minute ago, Namastyler said:

Thank you, yes, already done that too, no change.
I've even taken out the pcie-usb3.2 under it but there is no change.

what about swapping drives around (putting the one from mobo to the adapter)?

[Namastyler]

Just now, Mo5 said:

what about swapping drives around (putting the one from mobo to the adapter)?

Well yes, I could try that, but I have to unplug also my gpu to reach the first one and I have no idea if my bios will see the pcie adapter to boot the system at that point, so I was waiting to try something else first.
It's baking an ubuntu on usb right now, so first that, then if nothing works I'll swap too.

[Namastyler]

After many hours and fails I found the problem. Now I have 600MB/s in any direction.
If you have a similar problem, uncheck the "compress this drive to save disk space" under properties of the disk.
That's it. Really. I feel a bit ashamed but...