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#721
Do we take the "copy" comment literally? If so then this could be causing strange behaviour. Have you actually imaged to one of your internal sata drives and restored?
I have a very similar system configuration to you and your imaging time to a USB2 to a 5400 RPM ext HDD seems reasonable. Remember that you can find a significant difference in time depending on how full your ext HDD is. If it is quite full you will writing to the inner tracks and it will definitely be slower.
For interest, I imaged 50 GB (source) yesterday to a USB3 on a 75% full HDD and it took 12 minutes.