Chances are you're running a recent version of the firmware, in which case ypu must have formatted using EXT2.
That's quite an assumption. If you are using 1.08 firmware or newer you could have chosen EXT2 or EXT3. Before that, you could have only chosen EXT2 (apparently down to 1.02b when EXT3 was supported).
You can easily find out if you have telnet/SSH access to the box by issuing the "mount" command. This will list all mount points and will show the filesystems used. If you don't, there's another way by using a "fun_plug" file - but I'm not allowed to tell you how to do that, because it's against D-Link rules
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Another way, if you are brave, is to turn off your NAS, remove one of the drives and insert it in a PC and boot with a tool called GParted (http://gparted.sourceforge.net/) which will probably indicate the partitions and types on the drive. It will be the largest partition that you are interested in.