May I suggest you dig out the unit manual and read the section under Web UI --> Management --> Disk Management --> Hard Disk Configuration Wizard (page 38 in the manual we have). There you will find the procedure - critical step is Step 3 where you must be careful to select the correct drive to format.
NOTE: This is only safe if you do in fact have your disks setup as Standard (Individual stand alone disks) and NOT JBOD where if you format one disk you lose every bit of your data.
May I also recommend that you backup any data you don't want to lose from both disks.