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Full DD copy from hdd to hdd

If I have 2 of the following 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

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...