Assuming that it is a RAID1 configuration and that the RAID1 array occupies the entire volume of the drives (ie you did not have RAID1 & JBOD) - remove one drive, format the other, and then reinstall the first - you should end up seeing the two drives as Volume_1 and Volume_2.
If you try to change the drive configuration through the web interface it WILL format both drives.
If you have a RAID0 array then a backup and reformat is the only way forward.