A simple issue, but can be tricky nevertheless !

bareos-sd JobId 265: Warning: stored/mount.cc:270 Open device "FileStorage3" (/storage/block1) Volume "Full-0015" failed: ERR=stored/dev.cc:731 Could not open: /storage/block1/Full-0015, ERR=No such file or directory

While experimenting I had made the owner of /storage/block1 different from the Bareos user; This resulted in the storage deamon (bareos-sd) not able to make new files; Solving it, was chowning the directory and restarting the bareos-sd & bareos-dir :

chown bareos.bareos /storage/block1
systemctl restart bareos-sd
systemctl restart bareos-dir