external USB disk: 400MB/s transfer, but get only 40MB/s

Hello,

I did tests on two laptops (ASUS and C720) with different USB disks, and
writing with 

# dd if=file of=/mnt/file bs=8m

a big file of many GBytes, see below. The disk was always announced in
/var/log/messages as a 400.000MB/s transfers devices by the driver, but
the measured transfer was only between 40MB/s and 50MB/s. The file
system on the USB disks was NTFS.

Why is this?

	matthias


Jan 25 10:57:41 ASUS-150 kernel: da0: <WD Elements 2620 1023> Fixed Direct Access SPC-4 SCSI device
Jan 25 10:57:41 ASUS-150 kernel: da0: 400.000MB/s transfers
82594649636 bytes transferred in 2005.078575 secs (41192725 bytes/sec)


Jan 25 11:57:26 ASUS-150 kernel: da0: <WD Elements 2621 1026> Fixed Direct Access SPC-4 SCSI device
Jan 25 11:57:26 ASUS-150 kernel: da0: 400.000MB/s transfers
82594649636 bytes transferred in 1492.881431 secs (55325659 bytes/sec)


Jan 25 13:08:13 c720-1400094 kernel: da0: <WD Elements 2621 1026> Fixed Direct Access SPC-4 SCSI device
Jan 25 13:08:13 c720-1400094 kernel: da0: 400.000MB/s transfers
46271915961 bytes transferred in 1081.647154 secs (42779122 bytes/sec)


-- 
Matthias Apitz, ✉ guru@unixarea.de, http://www.unixarea.de/ +49-176-38902045
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