nanofrog can you tell me how to solve dual booting on Areca if I would like to boot OS X and Windows from RAID array ?
Currently I boot into Windows without any problems using default BIOS on Areca however both Windows and OS X array are visible on OS X too, but there is no way to boot from it (I have to use separate USB).
Is there a possibility to connect something small into one of built-in SATA ports into My Mac Pro and then just put boot off this drive and be able to select OS X or Windows on default boot screen ?
If you mean to boot them both from the same array using BootCamp, that's not going to work.
In past attempts, no one has gotten a RAID card to dual boot in a MacPro at all, single disk (pass through), RAID, or otherwise using separate or shared volumes either. Has to do with the RAID card having independent control of the drives (boot manager can't find the Windows installation).
This *might*, and I truly stress might, go differently now using Windows 8 (designed to work with EFI/UEFI systems), and I suspect would involve using a grub loader (or ReFit) to make it work. But you'd be a guinea pig at best (and have to try out all variations to see what does/doesn't work, such as separate volumes, ...), so this experiment is up to you.
The one way I know with certainty that you can create a dual boot MP (OSX + Windows), is to use a separate disk for Windows attached to one of the MP's SATA ports (2nd optical bay). Not exactly what you may have envisioned, but you will have native installations of both Windows and OS X (one on a single volume, the other on the RAID respectively).