If you want to add a second drive to an operational DNS-323, you have to be careful as there are few firmware bugs that can simply reformat your actual drive. In fact, all firmware up to 1.08 are known to be buggy.
The safe side is to format the new drive while removing the first one.
Another funny behaviour, is the fun_plug mecanism : Whatever was the position of your fist disk before inserting the new one, the drive holding the ffp will need to be inserted in the right slot !
Finally, you have to be careful that the drive inserted first on the system will be recognised as Volume_1. So to keep your first drive shares running, you will have to make sure that is is recognised first on the system, after the new drive has been formatted.
So, to add a new disk to your DNS-323, without risk, keeping fun_plug running and keeping your actual shares, you have to carefully follow few steps.