Hi,
my Asrock Q2900 with a software-raid works well, no reason to react ... but in german we have a proverb: Wenns der Kuh zu wohl wird, geht sie aufs Eis!
On the table lies a Adaptec ASR-6405E ... shall I change? Will it work w/ the MoBo? Will it dramatically increase both, speed and reliability? With a better look ...? 
Micha
The hw controller dies.....your raid will probably too. I would use it just to have more disks.
Personal opinion.
Maybe, hear they are very reliable, but never had one. What I mean also is that with software raid you can move your array to other machine independent of the controller.
I have though of.buying a sas controller just to increase the number of ports.
It is fast, but it does things its own way. If it dies you need to purchase a similar one to recover the RAID.
Do a test, just set it as JBOD (just a bunch of disks), and let the linux kernel deal with the RAID.
I think it should run worse though, as it has only a pcie 2.0 x1 interface.
You can probably find a dumb non-RAID SAS controller for cheap with a bigger pcie interface, then sell that RAID card.
No wait, I'm dumb. that's a pcie 2.0 slot, I was remembering wrong bandwith. 
That's 500 mb/s up and 500 mb/s down at the same time per line, and that's a x1.
4 hard drives won't really saturate that.
Should run fine even in JBOD, then. It is a bit overkill to run a true RAID controller in JBOD though, better if you use a dumb sata card or (better) a dumb SAS card.
with that controller set to JBOD you can make a 8-drive software raid, btw.
No differences on RAID1 afaik. (just test and see for yourself).
Hardware raid is better only if you are making RAID5-6 on a system that does not have hardware acceleration for cryptography/checksums (like that board you have). like AES-NI for Intel or just AES for some AMD APUs.
Although it's probably not a major difference on this scale.
If you want to use more than 2 drives it might be a better idea to run a RAID5, you get 3/4 ish the space instead of only 1/2 as with RAID1.
Nice calculation 3/4 of Space with costs for 3 HDs instead of 2 ... just kidding, I understand!
The fraction varies depending on how much drives you have in the array, I was kinda eyeballing the gains in your specific case where at most you could put 4 drives.
3 drives 2/3
4 drives 3/4
5 drives 4/5
and so on. The unavailable space amounts to the space of a single hard drive.
With RAID1 you have half the array space taken up by mirrored data, period.
Just tested the throughput in comparison:
2 diff USB3.0 Sticks, 1 Sata internal, Raid1 on internal Sata (Asrock MB Q2900) and on on Adaptec 6405e (w WriteCache)
Raid 1 internal from Raid to single HD:30 MB/s
Raid 1 Adaptec from Raid to single HD: 40 MB/s
Raid 1 Adaptec from Raid to USB1: 60Mb/s 
Raid 1 Adaptec from SATA on Adaptec to USB2: 100Mb/s 
->>>> Willllll haben!!!
Grüße aus Berlin
Micha
I think there is something wrong in your internal Sata BIOS settings or in OMV settings .
I'm getting better read/write speeds than your internal raid and single drives on a Zyxel nsa325 (a rather cheap and weak ARM nas hacked to run OMV).
Also look here, they report 80-100 mb/s of read/write speeds with a board like yours (a weaker processor) and an OMV software raid http://www.technikaffe.de/anle…ter_linux__openmediavault
See if he gives tips to improve your settings, I cannot read german beyond pre-school-kid level.
The Adaptec isn't faster, it's your NAS that is slower for unknown reasons.
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