Copy most recent backup for each selected source

[Dogwood]

I have a primary disk media set, "Main". It is now about 5.2TB

 

I have a 3TB drive I would like to copy only the latest backup for each source to. Up until now, a "Copy Backup" script using "Copy most recent backups for each source" has worked fine.

 

However, the size of the backups now exceeds the capacity of the 3TB drive, so the script fails. I would like to try using the Copy Backup option "Copy most recent backups for each selected source". However, I do not see any option to select specific backup sources. If I select that option then click Browse on the media set, it still presents a list of backups, not sources.

 

I may be missing something simple, but how do I select the specific sources, for which there are backups in the "Main" media set, that I would like to copy to the 3TB drive?