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An “offline” NAS can make a useful backup harder for ransomware on your main network to reach—but it is not a complete backup strategy, and it is not trustworthy until you have tested a restore. The key is to disconnect or isolate the backup outside backup windows, then keep another copy somewhere a fire, theft, or other site-wide event cannot take out with it.
Why keep a NAS backup offline?
A backup that is continuously reachable from the same network as your computers may also be reachable by malware or an intruder who compromises one of them. Ransomware variants can search for accessible backups and try to delete or encrypt them. CISA recommends maintaining offline, encrypted backups of critical data and regularly testing their availability and integrity in a disaster-recovery scenario. That is guidance, not a guarantee that any particular device or setup will withstand an attack.
Here, “offline” means access-isolated when the backup is not running—not simply a second NAS sitting beside the first. If the backup system stays connected and accessible, having separate hardware alone does not provide that isolation.
How to make a NAS backup offline
Disconnect it between backup windows
Schedule backups, then cut the backup NAS off from the network until the next run. You might do this by physically disconnecting its network connection or powering the device down, provided the method suits your equipment and backup workflow. Confirm that the backup actually completes before disconnecting it, and reconnect it only when you intend to run or verify a backup.
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Check any automated isolation feature carefully
QNAP documents a vendor-specific approach that disconnects a backup NAS outside backup activity. Its availability depends on supported hardware, firmware, and software; do not assume another NAS offers the same feature or that a feature name alone means the device is isolated. Check the manufacturer’s compatibility and setup instructions for your exact equipment.
An external USB drive is a simpler removable alternative: run the backup and then unplug the drive. It is only offline while disconnected, so leaving it attached to a computer or NAS defeats that separation.
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What an offline NAS protects against—and what it does not
Network isolation can reduce the chance that a compromise on a regularly connected device can also reach the backup. It does not protect against every event. A local backup may be lost in the same fire, natural disaster, or theft as the original equipment; Synology makes this limitation clear in its backup guidance.
Snapshots, immutable storage, network isolation, and off-site copies address different risks. A snapshot may help recover an earlier state, but it is not proof that a separate, usable copy exists. Immutability can make stored data harder to change, while an offline copy limits access during the periods it is disconnected. Neither replaces a copy kept away from the site or a successful restore test.
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Build a backup plan around separate failure modes
Synology describes the 3-2-1 baseline as keeping three copies of data, on two types of media, with one copy off-site. QNAP frames an expanded 3-2-1-1-0 approach as three copies, two media types, one off-site copy, one immutable or air-gapped copy, and zero recovery errors through verification. These are vendor-presented frameworks, not proof that following a label guarantees recovery.
Use the principles to check what each copy is meant to survive:
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- Network compromise: Is at least one backup disconnected or otherwise inaccessible outside backup activity?
- Device failure: Is there another copy on separate hardware or media?
- Site-wide loss: Is a copy stored off-site?
- Corruption or an incomplete job: Do you verify backups and periodically restore files?
- Recovery time: Can you retrieve the data quickly enough for your needs, and do you know how?
A second NAS can be the local isolated copy, but it does not by itself satisfy the off-site part of a broader plan. A removable drive can also provide separation, with trade-offs in capacity, handling, and convenience. The right arrangement depends on what you need to recover, how quickly you need it, and which losses you are trying to guard against.
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A completed backup notification tells you that a job ran; it does not establish that the data you need can be recovered. CISA specifically recommends regularly testing backup availability and integrity. Pick representative files and restore them to a safe location, then check that they open and contain the expected data. For a broader recovery plan, document how to restore the data if the original computer or NAS is unavailable, and test that process rather than relying only on a vendor’s verification feature.
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Keep the backup encrypted where appropriate, and store recovery credentials somewhere accessible during an incident but protected from the systems the backup is meant to outlast. If encryption keys or account access disappear with the original device, the copy may be unusable even if its files are intact.
Quick Recap
Sources and further guidance
- CISA, #StopRansomware Guide — recommends offline, encrypted backups and regular testing.
- Synology, Backup & Data Protection — describes local backup limitations and the 3-2-1 baseline.
- QNAP, 3-2-1-1-0 Rule — explains its expanded framework and vendor-specific backup-isolation approach.
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