By default the Backup Manager in Plesk stores local backups in /var/lib/psa/dumps.
You can change this location by editing /etc/psa/psa.conf. Find the following block in the file:
# Backups directory DUMP_D /var/lib/psa/dumps DUMP_TMP_D /tmp
Change the path for DUMP_D to your new desired location, then restart Plesk for the changes to take effect:
service psa stopall service psa restart
Alternatively, if you don’t want to make the change, you can mount another volume to the default backup directory (such as an Amazon S3 Bucket).