8 hard drives, very same model, software RAID0, terrible performance

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http://serverfault.com – I have 8 hard drives in my PC, assigned as storage. Three of them are attached to the mainboard. Four of them are attached to a controller [ SiL 3114 ] One of them attached to an another controller [ SiL 3114 #2 ] These hard drives are the very same, Samsung Spinpoint models. Same RPM, same size. I wanted to make a raid from ALL of them, using Windows Server 2008R2. However, the performance is just outright terrible. It's hardly faster than one hard drive. I checked the SMART values, they are all correct. Any ideas, what might be the problem? Ps.: There is a spare 320gb WD disk in (HowTos)