As @DavidPostill says, it'll be the physical drive itself that is the risk. Do not exclusively rely on SMART to warn you of impending doom however. Although pretty old now and referring to HDDs (i.e. magnetic drives), google's study did find that over a third of all drive failures were not preceded by SMART warnings.
I'll not go into software recommendations here as there's a huge world of continuously evolving choices out there, but this should give you an idea of what to consider when setting up a home backup system.
Common risks and threats to your data are:
- Overwriting / deleting your own files
- Security breach (which includes clicking the wrong link and finding your device and attached HDDs encrypted, with a hefty bitcoin ransom)
- Losing your drive encryption key
- Physical medium dieing
- Burglary, theft and loss
- Accidents and fire
For the first case something that does versioning would be desirable, so you can easily fall back to a previous version.
Also, in the first four cases, keeping a (regularly updated) backup NAS in the drawer (NOT! permanently connected to the computer) will also be sufficient. With the last two risks however, there's a high likelyhood that NAS will also be stolen / burned, therefore still losing you all your data.
You should therefore also consider an off site backup of some sort for critical data. This can be a cloud service (including a private NextCloud you can sync with), another NAS you backup to on a monthly basis and store elsewhere (trusted friends/family, bank vault etc). Note that SD cards, USB drives etc are not particularly reliable so it's better to get a removable HDD if you go this route.
Assuming that as with most of us cost is a constraint, you can choose to be selective in what you backup where, and at which frequency. My project working dir for example is backed up in several ways, so that I both have a remote version to fall back on, and a local one that I keep updated frequently (I'm planning to move this to a private cloud server which would make this even easier).
For the longest time I've also kept complete system drive images (updated every year or so) that made recovery after a system drive crash easier, however this requires a fair amount of space and thanks to changes in how windows activates may be less straightforward than it used to be. In case of Windows your best bet here is probably to use the Recovery Drive utility that's included with the OS. For MacOS I think setting up the Time Machine will sort this, after downloading a fresh MacOS copy you should be asked if you want to restore data from there.
Keep in mind privacy when selecting what to back up where. Clouds for example are not the place of choice for private documents (bank stuff, candid photos etc).
To summarise, backup system considerations are:
- Which data would have the biggest impact if lost? (digital family photos could belong to this category as well)
- Which data could be easily replaced? (therefore is less important to back up)
- Privacy considerations of each set of data
- Backup frequency (data that changes a lot and is critical vs data that only slowly grows over time and changes very little, and everything in between)
- Local vs remote backups (related to ease of recovery, off site backup upkeep and bandwidth availability)