Start with the size of the files you actually intend to protect—not your device’s advertised capacity—then allow room for backup history and future growth. SanDisk Support offers “about twice the size of the data” as a rough starting point for multiple backups and growth, but it is not a guarantee: full backups, long retention, and frequently changing files can require more. For reliability, also plan where copies live: the 3-2-1 rule calls for three copies, two types of media, and one copy offsite.
How to estimate the storage you need
There is no single capacity that fits every person. The amount depends on which files and devices you include, how much data changes, how many versions you keep, and how the backup tool stores those versions. A drive that holds one current copy may be too small for a backup plan that preserves older copies as files change.
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1. Define what you are protecting
Decide whether the backup covers selected folders—such as documents and photos—or a whole computer, including its other files. If you have several devices, decide whether you will back them up separately or include them in one destination. Write down the folders or devices in scope so you do not size the destination based on data you do not intend to save.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsExclude files only when you are comfortable losing them if the original device fails. A backup plan is useful only to the extent that it includes the files you would need to recover. SanDisk’s planning guidance starts with identifying what needs to be saved and totaling its size.
#1 Best Overall
- Easily store and access 2TB to content on the go with the Seagate Portable Drive, a USB external hard drive
- Designed to work with Windows or Mac computers, this external hard drive makes backup a snap just drag and drop
- To get set up, connect the portable hard drive to a computer for automatic recognition no software required
- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
2. Measure the selected data
Check the current size of the files you have chosen to protect. Use that figure rather than the computer’s or phone’s advertised storage capacity: the device may have unused space, system files, or folders that are not part of the backup. If you are combining data from several devices, add the included data sizes together, taking care not to count the same files twice if they are already stored in a shared location.
Record the measurement and date. It is a snapshot, not a permanent sizing answer; your library may grow, and a later backup may include data that was not present when you first measured it.
3. Allow for retained history and change
Ask how many older states you need to be able to restore. Keeping only the latest copy has different storage demands from retaining multiple versions of files or repeated full backups. The more history you keep, and the more data that changes between backup runs, the more space may be required.
Rank #2
- Easily store and access 5TB of content on the go with the Seagate portable drive, a USB external hard Drive
- Designed to work with Windows or Mac computers, this external hard drive makes backup a snap just drag and drop
- To get set up, connect the portable hard drive to a computer for automatic recognition software required
- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
Backup methods affect how that history is stored. A full backup copies the selected data; it is generally more storage- and time-intensive than methods that save changes. An incremental backup stores changes since a prior backup. The Canadian Centre for Cyber Security also distinguishes differential backups, which store changes relative to a full backup. These mechanics explain why one fixed multiplier cannot predict every backup’s overhead. That centre’s guide is written for organizations, so use it here to understand the methods, not as a personal requirement.
4. Add a growth margin and round up
SanDisk Support’s rough rule of thumb is a backup drive about twice the size of the selected data, to accommodate multiple backups and growth. For example, if your chosen files currently total 700 GB, twice that measured amount is 1,400 GB. That is a planning illustration, not a promise that a particular advertised drive will provide exactly that much usable space or that it will hold every retention scheme.
If you retain a long history, run repeated full backups, or have data that changes frequently, estimate those needs separately and plan for more than the rough starting point. Choose an available destination size above your estimate and leave headroom. The cited guidance does not establish a universal headroom percentage, so a precise percentage would be false certainty. Also verify the backup software’s storage and version-retention settings: the sources do not establish one universal amount of overhead for all tools.
Rank #3
- Easily store and access 1TB to content on the go with the Seagate Portable Drive, a USB external hard drive.Specific uses: Personal
- Designed to work with Windows or Mac computers, this external hard drive makes backup a snap just drag and drop. Reformatting may be required for Mac
- To get set up, connect the portable hard drive to a computer for automatic recognition no software required
- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
Why a reliable plan needs more than one large drive
Capacity answers how much data a destination can hold; reliability depends on whether a recoverable copy survives the same event that damages the original. The 3-2-1 rule is a way to plan that redundancy: keep three copies, on two different media types, with one copy offsite. The California Privacy Protection Agency’s 2026 consumer guidance recommends this arrangement and gives an example of originals on a device, a cloud backup, and an external hard drive.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Think of the three-copy count as a protection arrangement, not a formula that says to put three times your dataset on one device. A possible layout is your working files on a computer, a local copy on a separate external drive, and another copy in a different location, such as a cloud backup. A physically separate drive can serve as a local backup; an offsite copy addresses risks that could affect the computer and nearby drive together.
For a local drive to reduce exposure to network attacks, disconnect it when it is not being used for backup. A drive left attached can be exposed to incidents affecting the connected computer or network. A NAS can make files available across a household network, but network availability does not make it offline or offsite by itself.
Rank #4
- Easily store and access 4TB of content on the go with the Seagate Portable Drive, a USB external hard drive.Specific uses: Personal
- Designed to work with Windows or Mac computers, this external hard drive makes backup a snap just drag and drop
- To get set up, connect the portable hard drive to a computer for automatic recognition no software required
- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
Compare storage options by the job they do
Do not choose only by advertised capacity. Consider whether the option gives you enough usable room for your selected data and retained history, its one-time and recurring costs, its location and connection state, the speed and ease of restoration, and how it protects access to sensitive files. Check whether you can restore one file as well as a complete set.
| Option | Useful for | Tradeoffs to consider |
|---|---|---|
| External hard drive | Local copies, large files, or backup where internet access is limited; it can be disconnected after use. | Offers offline access when disconnected and can hold substantial data, but can be lost, stolen, physically damaged, or corrupted. Encrypt a drive that contains sensitive files. |
| Cloud backup or storage | An offsite copy and access across devices. | Depends on internet connectivity and account access; may involve ongoing costs. Review privacy and security settings, retention and version history, and how to export or restore data. |
| Network-attached storage (NAS) | A household with multiple devices or a large library that needs network access. | Network convenience does not make it an offline or offsite copy, and a network-connected device can be exposed to attacks. Pair it with a separate copy elsewhere. |
| USB flash drive or optical media | A small selection of files or an additional copy. | Physical media has loss and damage risks. Government archive guidance does not recommend flash drives as a long-term storage solution. |
These options can play different roles rather than compete as interchangeable destinations. For example, a disconnected external drive can provide local recovery without an internet connection, while a cloud copy can remain offsite. A NAS may be convenient for several household devices, but it does not replace a copy separated from the network or location. Actual cloud quotas, prices, retention policies, and hardware capacities vary; the guidance cited here does not compare current plans or products.
Keep copies separate, secure, and recoverable
Separate a copy from the original risk
Use different media types where practical and store at least one copy away from the location of the working files. The purpose is to avoid having every copy affected by one event, not simply to accumulate devices in the same place. An external drive kept beside a computer can be vulnerable to the same theft or physical damage; a drive disconnected after backup is less exposed to network incidents but is still vulnerable to local hazards.
Best Value
- High capacity in a small enclosure – The small, lightweight design offers up to 6TB* capacity, making WD Elements portable hard drives the ideal companion for consumers on the go.
- Plug-and-play expandability
- Vast capacities up to 6TB[1] to store your photos, videos, music, important documents and more
- SuperSpeed USB 3.2 Gen 1 (5Gbps)
Protect access and sensitive data
Encrypt sensitive files on physical backup drives. For cloud storage, consider encrypting files before upload when that fits the service and your threat model. Secure backup accounts with strong, unique passwords and multifactor authentication. These measures help protect copies that could otherwise be accessible through a lost drive or compromised account.
A synchronized folder is not automatically a complete backup. If a deletion or unwanted change is synchronized, the same change may affect other devices. Treat sync as a backup only if the service’s deletion recovery and version history meet your needs; check the actual service settings rather than assuming they do.
Check that recovery works
A copy you cannot restore is not useful protection. The Canadian Centre for Cyber Security recommends routine recovery testing in its organizational guidance. For personal use, periodically restore a sample file and confirm that it opens; also understand how you would recover a larger set if a device failed. A restore check can reveal missing folders, inaccessible accounts, or a backup process that did not capture what you intended.
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A practical checklist before you choose capacity
- List the devices, folders, and file types you intend to protect.
- Measure the selected data, not the device’s total advertised capacity.
- Decide whether you need only the latest files or older versions as well.
- Consider how often data changes and whether your backup method makes full or change-only copies.
- Use about twice the selected data as a rough starting point for multiple backups and growth, not as a guarantee.
- Increase the estimate for long retention, frequent changes, or repeated full backups; round up and preserve headroom.
- Plan three copies across two media types, including one offsite copy; assign each copy a clear role.
- Check the destination’s usable capacity, backup settings, security, restore process, and any recurring costs.
- Disconnect a local drive when appropriate, secure accounts, encrypt sensitive data, and test recovery periodically.
What the capacity estimate cannot tell you
A measurement and multiplier cannot produce an exact answer without details about your retention policy, data changes, software, and the way that software stores versions. SanDisk’s twice-the-data suggestion is explicitly a rule of thumb, not a measured guarantee. The 3-2-1 recommendation describes the number, type, and location of copies; it does not specify a personal drive size or a universal amount of storage overhead.
For a more precise estimate, use the backup program’s settings to determine whether it creates full, differential, or incremental backups and how many versions it retains. Then compare its expected storage use with your measured data and growth. The guidance cited here does not establish a universal formula for that calculation, so leave room and confirm that the backup completes and files can be restored.
Quick Recap
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