I pulled one of the drives during playback ( drive wasn't formated before). Then I pluged in the drive again. Unfortunately the SW marks the drive as bad and does not allow the same drive to be added again (system says that this drive is already  part of SW).

Solution:

1. Connect to DSS Console and open Extended Tools menu (CTRL + ALT + X).
2. Select option 14 - Delete content of units.
3. Select the Unit marked as a part of SW RAID (to select it use SPACE button), and hit enter to start erasing the unit.
4. Once information appears that system can be rebooted please press enter.
5. When system boots go to webGUI: SETUP -> Software RAID.
6. Mark the checkbox of the Unit you want to add back to the RAID unit, select "spare for MDx (RAIDx)" option in Raid level menu, and select Add button.