If I have [2 identical hard drives](http://www.pcworld.co.uk/gbuk/components-upgrades/internal-hard-drives/3-5-inch-hard-drives/wd-mainstream-3-5-internal-hard-drive-3tb-21651952-pdt.html) with the following characteristics:

* SATA 6.0 Gb/s
* 5400 rpm
* 3TB


connected to the same sata cable via a single sata port on the motherboard.  How long should a full dd copy take to complete?

So far it's been running for 5 hours and still going...

I am using Linux Ubuntu 12.04 64bit and the command I am using is:

    dd if=/dev/sdb of=/dev/sdc