Areca ARC-1100 writes really slow

[flying]

Today I installed an Areca ARC-1100 raid controller in my homeserver, the write speeds (tested with dd and bonnie++) are really low, read speeds are however fine.

Setup:
Motherboard: Gigabyte GA-MA78GM-S2H (Latest BIOS)
Cpu: Amd Athlon X2 5050e
Harddisk: Samsung EcoGreen 2TB
OS: Ubuntu Live from USB (also tried in Windows 7)

Speeds:
Read: 109 mb/s
Write: 23 mb/s

I tried both write-back and write-trough, it is a single disk setup with the areca in disk passtrough mode.

When I put the same Areca controller, same disk and same sata-cable in my other computer (Asus P5E, Intel E8400) the reads are the same however the writes are much higher (100 mb/s). The OS is exactly the same as on the homeserver (same USB drive).

Can someone help me solve this problem? Why is the write speed so low in my homeserver?

Thanks!