when your nas do its initial drive format it will create four partitions, a 532mb swap partition (if you have two drives it will make one on each but use only one), a 1gb hidden partition where it stores config files about shares and drive informations, then your regular data partition then another 1gb hidden partition not mounted during regular use, probably firmware backups or such.
to change how drives are identified you need to edit /dev/HD_a4/.systemfile/hd_volume_info.xml (HD_b4 for drive b)
you cant do it from web interface, you need fun plug installed (and a bit of linux knowledge), probably messing with volume info could break the other config files if they uses the volume id there stored.