This scenario only happens on SDs which were installed greater then a year ago.  The older SD configurations do not play nice with the new ReaR Linux BMR - specifically with extracting the ISO's from the SDs when downloading, thus creating a 0KB iso download.

the data however is still there and the ISO is available on the SD. 

PLEASE NOTE - newer SD installs will NOT have this issue

On your SD config file

replace {{****}} to appropriate values below and then restart SD service

Device {

    Name = TruncateDevice

    Media Type = {{devicename}} #Match other autochanger device media types

    Archive Device = {{arhivelocation}} #Match other autochanger device archive paths

    LabelMedia = yes # lets Bacula label unlabeled media

    Random Access = Yes

    AutomaticMount = yes # when device opened, read it

    RemovableMedia = no

    AlwaysOpen = no

    AutoSelect = no

}

Device {

    Name = RestoreDevice

    Device Type = File

    Media Type = {{devicename}}

    Archive Device = {{arhivelocation}}

    AutomaticMount = yes # when device opened , read it

    AlwaysOpen = yes

    RemovableMedia = yes

}

Device {

    Name = OldRestoreDevice

    Device Type = File

    Media Type = File

    Archive Device = {{arhivelocation}}

    AutomaticMount = yes # when device opened , read it

    AlwaysOpen = yes

    RemovableMedia = yes

}