In short, yes, you can undelete successfully. Your best chances for recovering your file(s) on a ext2 partition are immediately after deleting them -- your chances diminish the more you write data to the drive after deletion.
It's a pretty simple process: download a (.ISO) linux distro where you can boot from the cd/dvd drive (you dont want to (or need to) install linux on your pc hard drive. write the cd. connect your DNS321 drive to a sata port on your pc and you will be able to access your drive.
if you have a need for backup, in the future you might setup your DNS321 as 'standard' drives instead of RAID, and set the config for ftp such that hard drive2 (volume2) connects to hard drive1 (volume1) every night at 3am and makes a copy of the data.