Planning · The whole system
A 3-2-1 backup plan, worked end to end
The 3-2-1 rule is the shortest useful sentence in backup: keep three copies of your files, on two different kinds of storage, with one copy somewhere else. Rules are easy to nod at, though. This guide turns it into an actual plan for one ordinary home-office PC — with the file sizes counted, the drives chosen by arithmetic rather than vibes, and a routine that takes minutes a month.
Step one: count what actually matters
A backup plan starts with an inventory, not a shopping trip. Not everything on a PC needs three copies — Windows itself and your applications can be reinstalled. What cannot come back is your own material. For the PC in this plan, the count looks like this:
| Folder | Size | Why it counts |
|---|---|---|
| Documents | 45 GB | Work files, records, scanned paperwork — mostly small files that change often |
| Photos | 160 GB | Twenty years of family photos; irreplaceable, rarely edited |
| Videos | 110 GB | Phone and camera footage; big files, almost never changed after saving |
| Music | 15 GB | A ripped and purchased library |
| Desktop & misc | 10 GB | The working pile that never quite gets filed |
| Total that matters | 340 GB |
Do the same count on your own machine — right-click each folder, read its size, write it down. Every decision below hangs off that total, and guessing it is how people buy a drive that fills up in the second month.
The three copies, placed
- Copy 1 — the PC itself. The originals, on the internal SSD, where you work on them. This counts as one of the three copies, and it is the one most likely to be lost, which is the entire point of the other two.
- Copy 2 — an external drive that stays plugged in. A USB hard drive on the desk, written to automatically by File History every hour. This is the copy that handles everyday accidents: the deleted folder, the overwritten document, the drive that starts clicking on a Tuesday. Being always attached is what makes it automatic — and, as the failures table shows, also its weakness.
- Copy 3 — a second drive that lives somewhere else. A twin of the copy 2 drive, kept at a relative's house, a workplace, or anywhere that is not the same building. A cloud backup service fills this slot just as well; the two-drive version is used here because it needs no subscription and the swap doubles as a reminder the plan exists. What matters is the property, not the product: this copy is somewhere a fire, flood, or thief at home cannot reach.
Count the media types, because the rule asks for two: the internal SSD is flash storage, the external drives are spinning disks, and the two fail in different ways at different ages. (A cloud third copy adds a third kind — someone else's data centre.) Two drives of the same model bought the same day are still two copies, but spreading types and ages is cheap insurance against a bad batch.
Drive sizes, with the arithmetic
The backup drives must hold more than 340 GB, because a good backup keeps older versions, not just tonight's state. The arithmetic for copy 2:
- Current data: 340 GB.
- A 1 TB drive offers roughly 1,000 GB — about 2.9 times the data. That leaves around 660 GB of headroom for version history.
- Versions grow slowly for this file mix. Photos and videos — 270 of the 340 GB — almost never change, so they are stored once and then left alone. The churning part is Documents at 45 GB, and even a busy month of hourly versions of office files adds a small fraction of that.
- Result: a 1 TB drive serves for years before retention needs trimming. A 500 GB drive, by contrast, would start full and choke on versions within months — the classic false economy.
Buy the same 1 TB size for the off-site drive. The working rule to keep: a backup drive should be at least two to three times the size of the data it protects when it is keeping versions, and never smaller than the data itself plus room to grow.
Two external drives, labelled A and B. One is plugged in at home; the other is off-site. On the first of each month they change places. The drive that arrives from off-site goes straight onto the PC and becomes the current backup; the one that has been at home — holding a fresh, complete copy — goes off-site. The worst case this creates is precise and acceptable: a disaster at home costs at most the last month's changes, and only in the scenario where the home copies are both destroyed.
The routine
| When | What happens | Your effort |
|---|---|---|
| Every hour | File History writes changed files to the attached drive | None — automatic |
| First of the month | Swap drives A and B: the off-site drive comes home and is plugged in, the home drive goes off-site | Five minutes, plus the trip |
| Every three months | Restore drill: pick three files at random and restore them from the backup to a scratch folder, then open them | Ten minutes |
| Once a year | Recount the folders, check the plan's arithmetic still holds, replace any drive showing errors | Fifteen minutes |
The quarterly drill is not decoration. It is the only step that proves the restore path — the menus, the drive letter, the file versions — works before the day you need it under stress. Our companion guide walks through setting File History up and doing exactly that test restore.
What each copy saves you from
| What goes wrong | Which copy saves you |
|---|---|
| You delete or overwrite a file | Copy 2 — hourly File History versions, minutes old |
| The PC's SSD dies | Copy 2 — everything, up to the last hour |
| Ransomware encrypts the PC and everything plugged into it | Copy 3 — it was disconnected and in another building |
| Fire, flood, or theft takes the PC and the desk drive together | Copy 3 — up to a month old, which beats gone |
| One external drive fails quietly | The other two copies, and the quarterly drill is how you notice |
No single row of that table is exotic. Taken together they are the ordinary ways home and small-office files actually disappear — and each one is answered by a copy that already exists, placed there on purpose.
Sources
- Backblaze, The 3-2-1 Backup Strategy: Best Practices — the rule itself and its history.
- Backblaze, The 3-2-1 Backup Rule and Beyond — how the stricter 3-2-1-1-0 and 4-3-2 variants extend the basic rule.
- Microsoft Support, Backup and restore with File History — the hourly local copy used for copy 2.