Apparently it can use either which means you never really know.
Here is a description of how the drives are initialized on the DNS-323, maybe it
will help explain the behavior you are seeing.
In linux, the hard drives are assigned device names based on the order they are
detected on the hard drive controllers.
For the DNS-323 the hard drive controllers are labeled on the circuit board in this photo
http://wiki.dns323.info/_detail/11.jpg?id=start&cache=cache as HDD0 and HDD1.
In a single hard drive configuration, regardless if the drive is attached to HDD0 or HDD1,
the single hard drive will be detected and assigned the device name /dev/sda. The D-Link
scripts will mount the second partition on the /dev/sda drive to /mnt/HD_a2. The samba
configuration file is setup to assign a name of 'volume_1' to /mnt/HD_a2.
In a dual hard drive configuration, the hard drive attached to the HDD0 controller will be detected first and assigned the device namd /dev/sda. The hard drive attached to the HDD1 controller will be detected second and assigned the device name /dev/sdb. The D-Link scripts will mount the second partition on the /dev/sda drive to /mnt/HD_a2 and the second partition on the /dev/sdb drive to /mnt/HD_b2. The samba configuration file is setup to assign the name of 'volume_1' to /mnt/HD_a2 and 'volume_2' to
/mnt/HD_b2.