Hard drive enclosure recommendations

i have one of these http://www.xigmatek.com/product.php?productid=47 which is really usefull for what you want to do, it supports esata and usb 2.0 and takes 3.5 and 2.5 hard disks, there is a newer version of this which is usb 3.0.
been great had no problems with it, the good thing about these docks is you can just pop the hard disk in the top no opening it up and easy to swap drives about.
this is the usb 3.0 model http://www.xigmatek.com/product.php?productid=114

Jamstrange - I think you have the process backwards. When you get the dock, with the Duo off, resore your original good disk into the Duo and reboot it.

When it is up, connect the dock to your PC and insert one of the new drives in the dock and copy the files off the Duo onto that drive.

Then leaving the drive in the dock, shut down the Duo remove the drive and install the other new drive and set up the Duo. At this point you would have a 1 drive array on the Duo with one of the new drives but no data, and the other new drive in the dock would have a copy of the data.

Now, copy the files from the drive back to the Duo. When you are finished, remove the partition from the drive in the dock. This can be done with the disk managment application if you are running Windows. (In Win7 it's Control Panel -> System and Security. Then at the bottom of the window under the heading Administrative Tools, click on the link for "Create and Format hard disk partitions". You do not want to format the disk, but one of the options is to remove the partition by right clicking on the drive and then clicking on "delete volume".)

When the volume has been deleted, hot add the second new drive to the Duo and it should resync and when finished, you will have a redundant volume on the Duo. You can then reformat the old drive that is still viable, and install it in the dock and use it to backup file from the Duo.