your questions are not direct related to nextcloud. availability and backup/restore is more a linux question.
if you are concerned about future grow of your data use a volume manager (lvm) instead of attaching hdds directly to your server. google lvm for further reading.
if you are concerned about disk failure. again use a volume manager and raid.
if you want to do backup one starting point would be:
https://www.c-rieger.de/nextcloud-backup-and-restore/
or
restic can handle cloud storage as a backup target. so you write to aws s3, digitalocean spaces, backblaze, … (in combination with https://rclone.org/ your backup target get nearly infinite.) so your data are safe even if your home burns down. but 1tb cloud storage is not for free. 