Your correct about that being the local repo free or available space. Your overall disk space has to be able to generate the backup and to compress it prior to sending it to an external repository. You need to just add an additional disk if your NCS is a VM.
This link will explain the low disk space message and also how to mount a drive.
My guess is you're keeping too many copies of backups in the local repository...the default is 7.
Go to Administration>Background tasks then see how many copies you're keeping on the local repository.
If 7, reduce the number to a lesser value then remember to delete extra copies.
Hey, if you're 1st writing to a local repo then are sending to an ftp server via a script or manual process, why not just change the backups to write to the remote ftp server? All ya' gotta do is create an ftp repo then change Administration>Background Tasks>NCS Server backups to write to there.
Sorry I was a bit unclear. I have configured a ftp-repo on the NCS-server, which is the remote ftp-server. But as I have come to understand, the backup process on the NCS-server is configured (and not possible to change) to first make the backup locally and then send it to the ftp-repo. Because I have checked the ftp-server side and the ftp transfer does not start when the backup job starts.
So I have no script or manual process for the ftp transfer.
Regarding saved backups, there is no old saved ncs backups on the NCS-server.