Because you use driver-RAID5 on Windows, which does not offer any write combining techniques also slightly incorrectly called write buffering.
You need a good RAID5 driver, not available in Windows but only in Linux and BSD, to achieve high RAID5 write speeds. A real Hardware RAID controller like Areca would also allow proper RAID5 performance.
RAID5 is much more complicated than RAID0 and RAID1, or any combination of those. To achieve high write performance, you need a smart RAID engine. There are no smart implementations for RAID5 available on Windows. So you need a Hardware RAID controller if you want high performance RAID5 on Windows. Software RAID5 on Linux/BSD can give you roughly the same speeds, but at significantly reduced costs.
The best software RAID5 driver available on Windows would be Intel ICHxR "MatrixRAID" RAID5 driver with the "write caching" option enabled. It cannot compete with solutions available on Linux and BSD, however.
I couldn't find any Asus M2N-VMS motherboard, but i could find Asus M2N-VM, which is using a NVIDIA GeForce 7050PV / nForce 630a chipset. Any onboard RAID = Software RAID or also called Driver RAID; because all RAID processing is done by the Windows-only drivers on the host-system, which uses your CPU and RAM to do the RAID processing tasks. A *REAL* hardware RAID controller would do these tasks on its own processor and memory, and presents itself to the host-system as a SCSI controller that only requires a tiny driver to operate, the driver itself is only a "messenger" that acts as a bridge and does not perform any RAID-specific tasks on the host-system.
Driver RAID = Software RAID
There is nothing wrong with that, but none of the manufacturers ever got a good RAID5 implementation, they just do not understand RAID5 or ever bothered to write a proper driver. Good RAID5 drivers are available on both Linux and, even more so, FreeBSD.
Since you have nVidia RAID5 and not Intel ICHxR RAID5 driver, you can't even expect modest write performance. Also, writing this way will wear on your disks since they will be seeking excessively causing more-than-usual wear which may affect reliability and lifespan, and also causes higher power consumption, heat production as well as noise and vibrations. That is why you should not use your chipset onboard RAID functionality in RAID5 mode.
There is a way to get high speeds on dumb RAID5 implementations, but it only works if all the stars in the universe are aligned in just the perfect way; this trick is too complicated to work for end-users. If you really want to endeavour on this path go visit this thread: http://forums.storagereview.net/index.php?showtopic=257...