Performance problems with my NV+

$ dd if=/dev/zero of=test.bin bs=1048576 count=500
500+0 records in
500+0 records out
524288000 bytes transferred in 22.583006 seconds (23216041 bytes/sec)

--> 24216041/1024/1024 = 23MB/sec

So that poses a problem/bottleneck. I will reboot&fsck - which will
probably take forever - and I'm not sure if that will be of much help.

*EDIT*

I checked the Hardware Compatibility list : http://www.readynas.com/?page_id=82
My disks (ST31000528AS) are listed as supported for the NV+

I am a bit confused by what hdparm is reporting about the drives though:

$ hdparm -d /dev/hdc
/dev/hdc:
using_dma = 0 (off)

$ hdparm -i /dev/hdc
[...]
DMA: mdma0 mdma1 mdma2 udma0 udma1 udma2 udma3 udma4 *udma5 udma6
Cycle time: min=120ns recommended=120ns
PIO: pio0 pio1 pio2 pio3 pio4

So according to this udma5 is selected, but if hdma is to be trusted dma is still
switched off anyway ? Trying to set pio mode or dma on/off yields a "Function not implemented"