Backup drives are a low-stakes purchase that people overthink in the wrong direction. The specifications that get compared — transfer speeds, brand rankings, cache sizes — barely matter for backup. The things that do matter get ignored.
Here's what actually determines whether a backup drive does its job.
Capacity: buy at least twice your data
This is the most common mistake. Someone with 400GB of files buys a 500GB drive, and it's full within two months.
Backup software keeps historical versions. That's the entire point — it's what lets you recover a file as it was three weeks ago. Those versions need space. When the drive fills, the software starts deleting your oldest history, which quietly removes exactly the protection you were paying for.
Rule: at least 2× your current data. 3× if you're growing.
Check what you're actually using: on a Mac, About This Mac → Storage; on Windows, right-click your drive → Properties. That number, doubled, is your minimum.
Also consider: capacity is cheap and getting cheaper. The price gap between a drive that's adequate and one that's generous is usually small, and the larger drive means you don't repeat this purchase in eighteen months.
HDD or SSD?
Spinning hard drives (HDD) — mechanical, much cheaper per terabyte, physically larger, slower. Vulnerable to being dropped, especially while running.
Solid state drives (SSD) — no moving parts, faster, more expensive per terabyte, physically small, more shock-resistant.
For backup specifically, an HDD is usually the better buy. You're writing a large volume of data that you hope never to read. Speed matters once — during the initial backup — and after that, incremental backups are small. The money saved on capacity is worth more than the time saved on transfer.
Choose an SSD when: the drive travels with you, you're doing frequent full-image backups where speed genuinely matters, you're backing up over a fast connection where the drive would be the bottleneck, or the physical size matters.
One long-term consideration: for a drive that sits in a drawer for months at a time, spinning drives have a longer track record for unpowered data retention. SSDs are generally fine for reasonable periods but are not designed as long-term unpowered archival media. For a drive you're storing off-site and only connecting monthly, either works; for a drive you intend to write once and leave for five years, HDD is the safer choice.
Desktop or portable?
Portable drives — 2.5-inch, powered by the cable, no separate power supply. Smaller capacities, more expensive per terabyte, easy to carry to an off-site location.
Desktop drives — 3.5-inch, need mains power, larger capacities at better prices, and they stay on a desk.
For the off-site copy you rotate, portable is the obvious choice — you're carrying it. For the primary local backup, desktop gives you more capacity per pound and it isn't going anywhere.
A common and sensible setup: one desktop drive at home for the main backup, one portable drive that travels to the off-site location.
Connections
USB-C is the current standard and what to buy where you have the choice. Reversible connector, widely supported.
USB-A is the older rectangular connector, still on most computers, and perfectly adequate for backup speeds.
Thunderbolt is faster and more expensive. Unnecessary for backup.
The practical point: check what ports your computer has, and buy a drive that comes with the right cable or one that includes an adapter. A drive that needs a dongle you don't own is a drive you won't plug in.
Also check whether the drive is bus-powered or needs mains power, and whether the cable is captive or replaceable. Replaceable cables matter — cables fail more often than drives, and a captive cable failure means the drive is scrap.
Formatting, and the cross-platform problem
Drives come formatted for one system or another, and the format determines compatibility.
Mac backup with Time Machine: the drive must be formatted for macOS. Reformat it on first connection; the system will usually offer to.
Windows backup: NTFS.
A drive used with both: exFAT works on both systems for general file storage, but it's not suitable as a Time Machine destination and it's less robust than the native formats for large backups.
The honest advice: don't share a backup drive between platforms. Buy separate drives. They're cheap, and a drive formatted for one system doing one job is more reliable than a compromise arrangement.
Reformatting erases everything on the drive, so do it before you put anything on it.
How long drives last
This is the part worth internalizing: every drive will fail. Not might — will. The only question is when.
Large-scale failure data from data centres consistently shows drives failing across their whole life, with elevated rates in the first months (manufacturing defects) and rising again after several years. Consumer drives that live in a drawer and get used weekly generally last longer than data centre drives running continuously, but the principle holds.
Practical implications:
Replace backup drives every three to five years, regardless of whether they're showing problems. A drive is cheap; discovering your only backup is unreadable is not.
Don't buy two drives at the same time from the same batch for your two copies, if you can avoid it. Correlated failures are real. Buy from different retailers, or a few months apart, or different brands.
Watch for warning signs: clicking or grinding noises, files taking unusually long to copy, errors during backup, the drive disconnecting spontaneously. Any of these means replace it now, not later.
Check drive health periodically. Both operating systems and various free tools can read a drive's self-reported health status. It's not a perfect predictor, but a drive reporting errors is telling you something.
What doesn't matter much
Brand. Failure rates vary between models more than between manufacturers, and the data available to consumers isn't granular enough to act on. Buy a mainstream brand at a reasonable price.
Transfer speed specifications. Backup writes happen in the background. The difference between a fast and a slow drive is an hour of unattended time on the first backup.
Included backup software. Drives often bundle their own backup application. The built-in operating system tools are generally better supported and don't tie your backups to a vendor. Ignore the bundled software.
Ruggedized cases, unless the drive genuinely travels rough. For a drive that lives on a desk, you're paying for rubber.
Built-in encryption via a hardware button or fingerprint reader. These vary enormously in implementation quality. Software encryption from your operating system is better understood and more portable.
What to buy, concretely
For most people: two drives.
- One desktop HDD, 2–4× your data, for the local automated backup
- One portable HDD or SSD, sized for your irreplaceable pile, for the off-site copy
If you're only buying one: a portable HDD, 2× your data, and plan to add the second within a few months. One drive is better than zero, and it isn't 3-2-1.
Budget guidance: you should be able to cover both drives for less than the annual cost of a mid-tier cloud plan. This is the cheapest data protection available.
Setting it up properly
- Format it for your platform before putting anything on it.
- Label it physically. A sticker: "BACKUP A — [computer name]". When you have three drives in a drawer, you will not remember which is which.
- Note the purchase date on the label. That's how you'll know when it's five years old.
- Encrypt it. Both operating systems support encrypted backups. A backup drive is a complete copy of your life, and an unencrypted one that's lost or stolen is a serious exposure. Store the password in your password manager.
- Test it by restoring a file.
- Unplug it between backups.
That last step is worth repeating: an always-connected drive is reachable by ransomware. Plugging in weekly and unplugging afterwards is a small habit with a large effect.
Next in this series: How to Test Your Backup (Most People Never Do)