Move directory from one petition/device to another

[Cameron Beattie]

I would like to move a directory from one petition to another. For example 
/dev/sdb1 is mounted on /.
I would like to move directory /etc to /dev/sda2.

Could anyone offer advice on the easiest way to achieve this?

I have had a look at dd but that seems to allow for copying files or an 
entire device, not a directory.

Many thanks

Cameron