charlien
The Howto I referred to, also has information about backing up, restoring & transferring backup files from/to/via SSH between servers on the network, so read it completely as there will be a solution there to suit your situation.
In SME server server manager Backup or Restore panel, there are options to backup to & restore from workstation shares (which can be the workstation hard drive or a USB drive connected to a workstation, or a NAS etc, so you have plenty of options if a locally connected (to server) USB drive is not possible for you.
IIRC you can even connect/mount a SATA drive on the server to use for backup, I have done it.