Can someone help me trying to resolve this problem?
I've been trying to figure this out for years and haven't come up with anything.
So it's a known problem?
Known problem for some. I've never had an issue. Just read the Debian, Ubuntu, RedHat, etc forums and you will see reports. This isn't something OMV is causing. I've always thought the selection of hardware has something to do with it. I also think the number of times the server is shutdown affects it as well. Just another reason not to use raid (most don't need it anyway - I don't on most systems).
So there is no solution? In that case I'll need to send my hdd backs and buy a different model 
So there is no solution?
Don't use raid?? If I had a solution, why would I still be trying to figure it out?
In that case I'll need to send my hdd backs and buy a different model
Why? Format them each as ext4 and setup rsync to sync them hourly or something.
Don't use raid?? If I had a solution, why would I still be trying to figure it out?
Why? Format them each as ext4 and setup rsync to sync them hourly or something.
Maybe on some other forum someone found a solution.
I'll try one last time to build the raid, then I'll think about your solution 
thanks!
Maybe on some other forum someone found a solution.
Just to warn you... I've looked. There are plenty of solutions that work some of the time but none that work all of the time. Each time you sync that array, it is a lot of wear & tear on your drives as well.
Can you link me some solution, or suggest me how to find them? 
i'm searching for things like "linux mirror disappear", but I can't fiind much
Can you link me some solution, or suggest me how to find them? i'm searching for things like "linux mirror disappear", but I can't fiind much
You really are hoping this magically fixes itself. If only it were that easy. Important search terms are mdadm, missing or degraded.
Thanks! I found a lot posts now.
I found this one but I don't know how to do it:
What method to prefer, is up to you. I, personally, like the second but if you happen to have several (many) RAID devices and only want to start several of them at boot (required to have a working root filesystem) and activate the rest later, the first approach or a combination of them is a way to go.
I would try that since everything else didn't worked
I found this one but I don't know how to do it:
OMV doesn't use partitions for mdadm raid arrays. So, this doesn't apply.
Can you link me some solution, or suggest me how to find them?
Since this seems to an mdadm RAID issue:
Have you given thought to a ZFS mirror? I've been running a ZFS mirror for awhile and it's been trouble free. You're going to get other benefits that mdadm doesn't offer as well, like bitrot protection, self healing, etc.
If you have backup, now would seem to be the time to give ZFS a try.
There's a ZFS plugin, for OMV, and it works just fine. (openmediavault-zfs 3.0.18) If you install it, I did a little "How To" in a thread. ZFS Thread
Scroll down to "Setting up ZFS on OMV is pretty straight forward" and start there.
The only difference for you would be, when you get to the Create ZFS Pool dialog box, in Pool Type you'd select Mirror (for your 2x4TB drives).
As noted in the thread, after the pool is setup, I'd set up a monthly scrub to take full advantage of ZFS's self healing properties. The other stuff is, obviously, optional.
I mean, for what I now ZFS on Unix is a non official porting.
ZFS started on an Unix (Solaris), it has then be adopted by FreeBSD, Linux and others.
On Linux (you're talking about, not 'Unix') there is and has been a lot of confusion about licensing issues (see here for example) but technically ZoL (ZFS on Linux) works great, especially most recent version 0.7 and above (not usable with OMV yet).
And now that Oracle (who bought Sun together with ZFS and Solaris years ago) killed Solaris we might see ZFS on Linux rising even more.
First off, you are not on Unix.
Secondly, many packages have been ported between Unix like variants, one of the most widely ported being OpenSSH. I would venture to say that the number of installations using a ported version of OpenSSH far exceeds the number running on the Unix like variant it was originally developed for.
For stability and security reason shouldn't it better to use ext4/btrf over zfs if I want to remain on unix?
After looking it over the ZFS Thread, when you "edit pool" properties, you might not want to turn compression on. It wouldn't hurt anything to turn it on and it might save some disk space. Either way, your call.
When you mentioned security above, the other pool edits are done to duplicate (more or less) Linux file/folder permissions. For all practical purposes, after the edits, security will be the same on the ZFS pool as it would be in ext4.
BTW: I passed up BTRFS RAID1 because it's not stable and it's unlikely to be for the next couple years. If you want a functional equivalent of mdadm RAID1, that is stable, a ZFS mirror is the only real choice available.
I passed up BTRFS RAID1 because it's not stable and it's unlikely to be for the next couple years
Your claim is based on what exactly?
Btrfs' RAID-1/RAID-10 is stable since ages it just needed more than 2 devices since with a simple two disk mirror and one disk failed the mirror went read-only. Problem known, use 3 disks, done.
According to official docs this has been fixed with 4.13 now, status is 'OK' while read performance still could be improved: https://btrfs.wiki.kernel.org/index.php/Status
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