Try making a backup first on that VPS, then upload the existing one.
def has_enough_space_on_disk?(size)
`df -Pk #{Rails.root}/public/backups | awk 'NR==2 {print $4 * 1024;}'`.to_i > size
end
This function fails when the public/backups directory does not exist (making a backup creates it).
Maybe add another check?