weird throughput on write to SATA disk

[kenneth johansson]

When writing data to /dev/sdb (that is the whole disk and no filesystem)
it starts out ok around 100MB/sec but soon end up doing considerably
worse. For prolonged times lasting several minutes it hovers in the
20-30MB/sec. 

This happens on two different SATA disk one samsung and one seagate and
on both 2.6.26 and 2.6.27 kernel. The disk are connected to the external
esata connector. Controller is 00:1f.2 SATA controller: Intel
Corporation 82801HB (ICH8) 4 port SATA AHCI Controller (rev 02)

What could be the reason for this ?? reading is no problem.

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