An Internal HDD has the advantage that you can see your data safely all the time, but I'd think that storing the data on an external HDD in a cabinet, or somewhere safe place probably makes the HDD live longer. Is that so? Or not really?
I have two SSD in both my PC and my laptop. And two DAS connected to the PC and shared over wifi. One of the DAS hold my large media and backups. The other DAS is mostly used for backups.
Every time I boot my PC, or laptop, a new rsync snapshot of the full /home on the primary SSD is automatically created on the secondary SSD.
I also manually create rsync snapshots of folders on the PC/laptop primary on the primary DAS and also of folders on the primary DAS to the secondary DAS. By manually I mean automatically, but only after I trigger it by double click on a script on the desktop.