Don't get these write figures on the hard drive...
I've got a 15K Hitachi SAS drive (HUS151414VLS300) on my Dell 690 machine. I've ran ATTO Disk Benchmark what it shows is that the write speed on small file sizes is virutally non-existant (although the read speed is fine) and on large file sizes it's normal. It's only on this drive that I seem to have these problems. I've got other SAS drives connected to the same controller and their fine. Other external/internal drives are no problems either. There is no write cache enabled biut it isn't enabled on the other drives either.
Here's a link to the results: .
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Would you know what mobo your Dell uses? And its specs? What about your other components?