If we ignore the other problems and the HDD size is the only problem, you can use duply as backup engine. It's an incremental backup tool with backup encryption, so if the user not change the whole content all day, only the first backup's size is 5GB and the rest is a couple of megabytes, because it's same as in the first backup. So you can store the backup for 30 days, and make a full backup every month. I use this method on all of my servers. Only the SQL needs to be saved all time (Because without binlog replication it's not support incremental backups).
