It is my understanding that the main compatibility issue with the "EARS" drives (WD10EARS, WD15EARS, WD20EARS, etc) is that the partitions and file systems must be lined up with the 4K sectors they use internally, otherwise you will experience suboptimal read/write performance. If it not lined up the drive will "work" but will be slower as each file system block/cluster access (which by chance is 4k in size on most systems) will require accessing two of the internal 4k sectors instead of just one.
Annoyingly it appears that the DNS-321 currently does not know how to make this adjustment, and the jumper these drives use to offset the simulated 512byte sectors by one is a hack that may or may not work depending on the partition's location. There may not even be a way for OS software to automatically detect if a drive uses 4k sectors (to the OS and controller hardware the drive pretends the sectors are 512 bytes for compatibility)
The worst part is that large low-power hard drives without the 4k sectors are harder and harder to find. Even the other manufacturers are using 4K sectors drives that require this special alignment. According to various specs the Western Digital Green "EADS" drives (WD10EADS, WD15EADS, WD20EADS, etc) still use 512 byte sectors and appear to be intended for NAS or embedded devices, but they are more expensive and less common.
The 2TB WD20EADS is actually listed as a supported compatible drive for the DNS-323 (should apply to DNS-321 also) on this thread:
http://forums.dlink.com/index.php?topic=672.0 although it looks like that list has not been updated in a while. Logically the 1TB WD10EADS and 1.5TB WD15EADS models should also be 100% compatible.