Syncing is not backup. The widely used backup rule, automating it, testing a restore, what to back up first and the weakness of always-connected drives.
Cloud sync keeps the same files visible across devices. Delete something by mistake or let a file corrupt, and that state propagates to the other devices too. A backup is a backup only if it can return you to an earlier point in time. Using a sync service does not mean you have backups. Some sync services offer a bin or version history, but these usually keep items for a limited period, so they cannot be relied on to bring back a point from long ago.
Keep three copies, on at least two different kinds of storage, with one of them physically elsewhere. Fire or theft takes every copy in the same location together. For an individual, the original plus an external drive plus cloud storage is generally enough. Using an external drive alongside cloud storage satisfies both the media and the location conditions at once.
Backups that rely on remembering are the first thing dropped when you are busy. Turning on the automatic backup built into your operating system or cloud service, so it runs on a schedule, is what lasts. Backups to an external drive can usually be set to run automatically when it is connected. Automatic backups can also stop silently when storage fills up or a setting changes, so check the date of the last successful run now and then.
It is not rare for backup files to be corrupt, or for a misconfiguration to mean nothing was ever saved. Finding out on the day you need it is too late. Occasionally restoring a few files and checking they open is the only reliable verification. If possible, test the restore on a different device, because the scenario to plan for is the original one failing.
Trying to back up everything often ends in backing up nothing. Start with what cannot be recreated: photos and video, documents you wrote yourself, certificates and recovery codes. Installers and anything you could download again rank lower. Data stored inside apps, such as messenger chat history, often needs the app's own backup feature switched on separately. Writing down once what you back up also means nothing gets missed when moving to a new device.
Ransomware, which encrypts files and holds them hostage, can also encrypt external drives and sync folders attached to the computer. Leaving the backup drive plugged in all the time means the original and the backup fall together. Connecting the drive only while backing up and disconnecting it afterwards, or using a method that keeps earlier versions from several points in time, provides protection. Restore points and version history built into the operating system help too, but if they sit on the same disk as the original, they vanish with it when the disk fails.
No storage medium lasts forever. Hard drives have moving parts and suffer from shocks and age, while SSDs and memory cards left unpowered for long periods can become unreliable at holding data. Moving backups to fresh media every few years is the safe course. Cloud accounts can be locked or services shut down, so they should not hold your only copy either.
Backups are easy to set up and forget, so running through these items about once a quarter is enough. It is also worth a check after buying a new device or starting an important project.