USB 3.0 slow on 12.04 64 bit

I'm using an USB 3.0 external hardisk Toshiba model HDTB105EK3AA on my Ubuntu 12.04 installation. It works fine but the transfer rate is around 28 MB/s both reading or writing. Is it the right transfer rate? The expectation was over 60 MB/s
The internal hardisk is Sata II and the only activity on the system is the copy to usb disk.
The motherboard is ASUS F2A85VPRO and CPU is AMD quad core A85600K.
This is the result of lsb_release -a; uname -a; lsusb
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 12.04.2 LTS
Release: 12.04
Codename: precise
Linux Sala 3.2.0-37-generic #58-Ubuntu SMP Thu Jan 24 15:28:10 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 006 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 007 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 008 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 009 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 010 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 002: ID 093a:2510 Pixart Imaging, Inc. Optical Mouse
Bus 010 Device 002: ID 0480:a006 Toshiba America Info. Systems, Inc.

and this is the dmesg | tail after mounting the USB hardisk.
[ 4805.343717] usb 10-2: new SuperSpeed USB device number 3 using xhci_hcd
[ 4805.361695] scsi9 : usb-storage 10-2:1.0
[ 4808.566291] scsi 9:0:0:0: Direct-Access TOSHIBA External USB 3.0 0101 PQ: 0 ANSI: 6
[ 4808.567385] sd 9:0:0:0: Attached scsi generic sg2 type 0
[ 4808.596448] sd 9:0:0:0: [sdb] 976773164 512-byte logical blocks: (500 GB/465 GiB)
[ 4808.612235] sd 9:0:0:0: [sdb] Write Protect is off
[ 4808.612244] sd 9:0:0:0: [sdb] Mode Sense: 1f 00 00 00
[ 4808.623156] sd 9:0:0:0: [sdb] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ 4808.725111] sdb: sdb1
[ 4808.839539] sd 9:0:0:0: [sdb] Attached SCSI disk
ricky@Sala:~$ ^C

Many thanks!
Ricardo