Before you do anything - BACKUP the data.
Next - how as your RAID1 configured? Was the entire 1.5TB used to form the array or just a part of it?
If the entire disk space was used to form the array, there should not be a Volume_2, but if you did not use the entire disk for the array, there WILL be a Volume_1 & a Volume_2 and the data written to one will not be written to the other.
Check the status page, does it tell you that there is a RAID array and if so what is the status shown?
Assuming that you DO need to fix the array - I would SUGGEST the following - please not this is just a suggestion, I have not personally experienced the problem you describe and cannot offer a proven solution.
Remove a disk from the DNS-323 and power it up - it may detect a drive failure and indicate this with an amber LED, and a degraded status - power it off.
Install the disk in a PC and use diskpart (assuming XP, Vista or Win7) to remove all the data and partitions, and then reinstall it in the DNS-323 and power it up - it should now detect the "new" disk and prompt you to format it, if the checbox to create a RAID array is available, check it.
What we are attempting here is to force the unit to either treat the disk as a replacement for a failed disk in an array - or - create an array where there is none (or where it thinks there is none.)