If you are not sure , I strongly advise you make a backup of your data on an external storage for worst case scenario.
OK, in general when setting Failover data on backup target NAS will be erased. Your the picture shows LT-LANDISK2 is going to be registered as backup Device. This means if you continue with setup all data on LT-LANDISK2 will be erased
All data on LT-LANDISK1 will remain intact.