Sync Is Not the Same Thing as a Backup
A synced folder mirrors changes, including deletes and mistakes. A backup keeps a copy that does not follow every error.
A folder that appears on your laptop and your phone feels like a backup because the files are in two places. Often it is sync, not a backup. Sync tries to make the copies match. If you delete a file, edit it badly, or a program damages it on one device, the service's job is to carry that change to the others. A backup's job is different. It keeps a copy that does not have to follow you off the cliff. The distinction is simple and expensive to learn on the day you needed the old version.
Matching is the feature and the hazard
Sync is genuinely useful. You start a note on one device and continue on another. The hazard is hidden in that convenience. There is often one current state, replicated, rather than a protected earlier state. Emptying a folder, or a buggy app that rewrites files, can empty the replicas too, sometimes before you notice. A spare drive that you update on purpose and then unplug will still have yesterday's folder after today's mistake. The synced folder may already have learned the mistake. If you have ever been grateful that a recycle bin still held a file, remember that sync may empty the remote side as diligently as it copies the good days.
Deletes travel unless something keeps history
Some services keep deleted items for a while, or keep older versions of a file. That history is closer to a backup, and it is only a backup if you know it exists, you know how long it lasts, and you have restored one file as a test. A version history you have never opened is a rumor, like any other untested copy. Read the setting before you rely on it. If the window is short, a mistake you notice next month is outside the window. An unplugged drive does not expire its older files on a vendor's schedule. It sits there until you overwrite it. That patience is the point of a spare you control.
Sync does not leave the building by itself
A sync folder among your own devices, all in the same house, shares the house. It can be an excellent way to move files and a poor way to survive a fire or a theft of every device that was signed in. Offsite backup is a separate question, answered by what offsite means and by the questions you ask before trusting an online service. A service can provide the elsewhere copy. It does that only if the files are actually there, you can download them, and a delete at home does not immediately become a delete in the account. Ask that question directly. The word cloud does not answer it.
Use sync for work in progress, backup for the record
Let sync carry the draft you are actively editing, if it helps you. When a folder becomes something you would grieve, copy it to the spare in the ordinary way, as a visible folder, and log the date. The synced version can keep changing. The spare can lag a little and remain the place you return to. This is the same split as a master and a working copy, applied to locations instead of edits. People get hurt when the synced working set is the only set. The newest files are the ones most likely to exist only inside the sync, because that is where they were born. Give them a second home that does not auto-follow.
Test a delete before you trust the feeling
Make a harmless file, let it sync, delete it on one device, and see what the other device and the website do. Restore it if you can. That experiment teaches more than the product's adjective. If the delete raced ahead of you, you now know you need a spare that is not part of the sync. If a version came back easily, you know the steps, and you should write them in the backup note while you remember them. Either result is a gift you gave yourself on a calm afternoon. Sync is a delivery system. Backup is a memory that does not have to obey the latest instruction. You are allowed to use both. You are not allowed to call them the same thing and then be surprised.
Frequently asked questions
If I have three devices in sync, is that three copies? Not in the 3-2-1 sense, if a single delete or a single account failure can reach all three. Count a synced set as one copy unless you have tested that older versions survive a mistake.
Should I turn sync off? Not necessarily. Use it for what it is good at, and add a real spare for the folders you cannot replace. Turning it off without a spare leaves you with one device and a feeling of purity.
What about a folder I marked as offline files? Offline files still follow the account's rules when they reconnect. They help when the network is down. They are not, by themselves, a backup that resists deletion. Test them the same way.