Ramses, in my search to find a solution, I've reistalled the firmware for my FF400 and I discovered that it is very old, from 2010 and it supports only until windows 8.
This might be the answer, why your UFX works with windows 10 and my FF400 doesn't.
Despite that, do you think it might be a Cubase problem?
It does the same everytime I tested it.
It plays back only the first track I record, even though the waveform is visible on the second track too.
If I record a third track, sometimes it plays back normaly.

No, no, everything is ok in that department.
I know what I'm doing.
When I say it does not play back properly, I mean that I don't hear what I've recorded, only white noises and glitches.

By "unit" you mean the FF400?
How can it be faulty, if it works fine with my older computer?
Perhaps RME's windows 10 drivers are faulty for my FF400.
Can anybody from RME comment on that?
No answer yet, to the email I've send them, explaining my issues thoroughly.

Well I have news.
I tested my setup with a tascam usb audio interface and I have exactly the same problem.
I installed Sonar Platinum and tried it.
Zero problems with either the FF400 and the tascam!
I installed Cubase 8.5 in a laptop with windows 7 and recorded in an internal SSD drive.
Zero problems there too.
I recorded in my SSD programs drive.
Zero problems.
I opened the problematic(wich didn't play in Cubase and only white noise and glitches were audible) audio file with Windows media player and it played back normally.
I saved the project and I opened it again.
Now the first track had the same problems and not the second.

Conclusion until now:
Something is wrong with Cubase 8.5 and windows 10 handling audio recordings and hard disk drives, or some conflict between Cubase and my system.

FF400=OK!
EXSYS EX 16415=OK!
PC=OK!
Windows 10 with Cubase 8.5=?

Go figure...

I have absolutely zero issues with Windows 10 and Cubase 8.5.20 and haven't since I installed Windows 10, last July, a full year ago.  I also have zero issues running my Fireface 802 in Windows 10, in firewire mode, through Cubase 8.5.20.  Windows 10 is far from being the black sheep it seems to be made out to be.

How old is that WD drive?  It is either not up to par for realtime recording/playback, on its way out health wise, or the way it is configured for either caching, power configuration etc, is off and hindering its performance.  The fact that the faster SSD works fine, is saying a lot here.  The culprit appears to be that drive, as it acts up when the project is copied to it, yet everything else stays the same.  OS, DAW, drivers etc.

Is there some special configuration needed, for Cubase to use them properly? Because let's not forget that Sonar Platinum wich I've tested has no problem recording fine on them.

you don't have to rename your preference anymore.  Cubase has the option since version 7, to temporarily start it up in safe mode/fresh preferences for testing purposes.  After you exit, it goes back to loading full preferences as before, on next start up.

https://www.steinberg.net/forums/viewtopic.php?p=190932

With the new motherboard the problem is still the same!!!
I don't know what else to do.
They said they tested the hard drives and they work fine.
So what happens now?
I have the new pc for two months now and I can't use it for music.

The new motherboard has a new chipset.
It's not the same.
The type is the same, but that cannot be changed, only if I downgrade to an older system proccesor, ram, etc...
Is there a chance, the hard drives show up OK in the tests, but are not OK in reality?
Windows 7 is the only other option I've not tried yet.
Could there be a conflict with some other hardware, like my graphics card?
I have to exclude something in the end.
Hardware first.

I don't understand why a new motherboard chipset can not support simple all time classic hardware like hard drives and to be more specific WD black caviar, wich are supposed to be one of the fastest and best overall.
This doesn't seem logical to me.
Afterall my problem is very strange.
2 tracks recorded, the second not playing back normally, only white noises and glitches, but only if I record on the WD's.
No problem with the SSD, or an external USB drive.
What has that to do with the chipset of the motherboard?
Aren't all motherboards build for using hard drives anyway?
I never had this problem in my recording life!!!

I'll say it again : I was informed that Z170 and H170(inferior) are the only types supporting I7 6700 1151.

OK I've read it.
They say that their systems were under heavy stress playing the latest games.
Are 2 audio tracks with no plugins "heavy stress"????
Unbelievable!
Why does the problem disappear when I record on the SSD, or on an external drive???
Doesn't the Z170 chipset count there?

I feel something is up with the drives.  I know you said they were tested and came up clean, but perhaps they still aren't up to snuff for multi-track recording.  The fact that the same motherboard and chipset works fine with the SSD drive and an external drive, has me believe the motherboard and chipset are fine.  If absolutely no hard drives recorded, I would look elsewhere, but the fact that you do have working hard drives without issue and drives that do show issue, shows me the drives are either not up to snuff, or perhaps there is some sort of compatibility issue with those particular drives on those ports.  I would look into different drives, from different manufacturers, different model, different size etc and find something that works.  Keep the SSD for your boot drive and apps, but find more suitable drives for recording too.

First try this http://www.win-raid.com/t2f23-Intel-RST … -WHQL.html Read it all and start with the one he advices for your chipset. If it doesn't fix it it is either the sata cable (first suspect) or the HD itself. I use 3 blacks without problems, but for my sample libraries.
Good luck!!!!

I remember something I had too with my black drives. Try connecting them one by one. One of my black drives caused all kind of issues in combo with the others. Not solo on another system.

If you not already do that, http://www.gigabyte.us/products/product … D=0#driver   install the sata/raid driver.

Any more details? You mean 3 together?

Does that mean, we're back in the chipset department and the firewire pcie card?
Could it be the I7 6700K?
RAM?
Windows 10?

To see if it is firewire/rme problem. Try onboard audio with asio4all driver. Don't change the latency on the asio4all driver, it is buggy that way, but it gives an indication if the mobo can work for audio.

Is there a chance, the RAM speed could be responsible for my audio stutters and dropouts?
The technician who changed my hard drives told me that he managed to clock the RAM speed to 2400 and I've read somewhere that  my system can handle only 2133.

Overclocking in itself isn't a problem, it is just being sure the parts are stable at said clocks. Stock speed parts can cause just as many problems as overclocking parts, if they too aren't stable. I have my parts overclocked from day 1. My 3.4ghz CPU is very happily running at 4.5ghz and has for 5 years now, just be sure the parts can reliably run at those speeds. You can certainly try and put your RAM back to stock speeds and check, but most likely the ram is fine, as they are designed to work above their rated spec out of the box.

Is this a 100% clean install of Windows 10 by chance?  No upgrade and made on that exact motherboard?  If not, a clean install may be in order.

My pc is completely new(2 months old) and windows 10 are clean installed there from the start.
My RAM speed is 2400, but I've read that skylake I7 6700K 4GHZ(that's my proccesor) doesn't support speeds above 2133 and that it is preffered to underclock, to match the system.