NSA320 process to replace a bad disk in RAID 1

Mijzelf

The 'source disk' is 'disk 2', which is at the moment the only copy of your data. (More or less. If the repair indeed succeeded to 99.999% it should be possible to mount it, when your 'disk 2' dies now).

A new volume will delete the current content. And no, you don't have to backup it, as the NewDisk doesn't contain any unique data.

Is there a method or specific steps to "copy the data over, and finally add the old disk to the new volume"? 

Well about adding old disk to new volume, here you can read about converting JBOD to RAID1. When you have created a JBOD volume on NewDisk, and you plugin 'disk 2', I *think* the box will have 2 volumes (it is a good idea to give the new volume a different name), in which case you can copy the files over using the File Manager in the webinterface. It can also be done between shares, but that will be awfully slow, as each file has to be downloaded and uploaded again.