To expand on what FurryNutz said.
The DNS-320L, like almost all small NAS boxes use software defined RAID (RAID in this case includes single standard disks) and as such needs to format the disks to write that structure. The actual format for the file system is the Linux Ext3/4/5 format.
It is possible to move a set of disks from one NAS to another IF the NAS boxes are the sane brand and series with the same firmware and still maintain the integrity of the data (even then it is recommended that a full backup be made in case of problems).
Disks that have been formatted by the windows operating system are NOT compatible with most small NAS boxes and need to be formatted in the NAS when they are first installed.
In this case I assume these disks are from a computer. If that is the case then you need to make a backup of the data on them (even if the computer was Linux based) before installing them in the NAS and setting them up there. You then use that backup to restore the data.