I believe I have found a bug when converting from a single drive to a RAID1 pair (as I discussed in my thread at http://forums.dlink.com/index.php?topic=12964.0). I don't know which firmware version this bug appeared, but in my current 1.08 config:
When converting a single drive configuration to a RAID1 pair, the right drive is always formatted, regardless of which drive has data and which drive is new.
This was with 2 identical drives, bought at the same time with serial numbers within close proximity to each other.