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For most two-bay NAS owners who need the system to keep working after one drive fails, start by evaluating a supported mirror: RAID 1 or the manufacturer’s equivalent (such as a two-drive variant of a proprietary RAID system). A mirror normally gives single-drive failure tolerance, but usable capacity is limited by the smaller drive. It is not a backup. Add an independent, versioned backup to an external USB drive, another NAS or cloud destination, and perform a real restore test.
The exact RAID choices, filesystem, snapshots, migration options, drive compatibility and capacity depend on your NAS model and operating-system version. Use the workflow below to make those model-specific checks before creating the storage pool.
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Choose for the failure you are trying to survive
A two-bay NAS can address two different goals:
- Availability: keeping shared files and services online when a drive fails.
- Recoverability: getting files back after accidental deletion, corruption, ransomware, theft, fire, a failed NAS, or an administrator mistake.
RAID primarily addresses availability. Backups address recoverability. Because the same files, deletions and corruption are usually written to both members of a mirror, two drives in RAID 1 are not two independent backups.
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| Priority | Starting point | What it helps with | What it does not solve |
|---|---|---|---|
| Continue operating after one drive fails | RAID 1 or the NAS maker’s supported two-drive mirror | Drive-failure downtime and replacement of one failed member | Deletion, ransomware, theft, a failed NAS, or damage affecting both drives |
| Recover an older version of a file | Versioned backup and, where supported, snapshots | Accidental edits, deletion and some ransomware events | A backup that was never completed or cannot be restored |
| Recover after loss of the NAS itself | Independent local and offsite backup copies | Theft, fire, flood, controller failure and total device loss | Instant replacement of the NAS hardware |
What RAID 1 means in a two-bay NAS
Usable capacity follows the smaller drive
A mirror writes the same data to both drives. The usable space is therefore approximately the capacity of the smaller member, less filesystem and system overhead. For example, pairing a 12 TB drive with an 8 TB drive does not produce 20 TB of usable mirrored space; the larger drive is generally constrained to the smaller drive’s size. The exact displayed capacity depends on decimal versus binary units, filesystem reservations and the NAS implementation.
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Do not rely on a generic calculator for an exact result. Check the storage manager for your model, drive sizes and chosen filesystem before committing data.
One-drive failure tolerance has conditions
A supported two-drive mirror can continue serving data after one member fails, but it is then running without redundancy. Replace the failed drive with a compatible unit and allow the system to rebuild. During a rebuild, performance can fall and the remaining drive is carrying the only copy inside the array. A second failure, an undetected read error or an administrative mistake can still cause loss, so start or verify an independent backup before replacing a drive whenever the data is not already protected elsewhere.
Do not select RAID 0 for important data
RAID 0 stripes data across both drives and provides no drive redundancy. A single drive failure can make the array unavailable. It can be appropriate only for disposable data where performance or combined capacity matters more than recovery, not for a two-bay NAS being configured for protection.
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Some manufacturers offer a branded layout that can use two drives as a mirror while allowing different expansion or migration rules. Synology’s product family, for example, includes SHR, but its behavior depends on the model, DSM release and drive arrangement. Treat any branded option as a manufacturer-specific implementation: read the current storage-manager documentation for the exact NAS instead of assuming that every two-drive configuration behaves like every other one.
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Check these facts before creating the storage pool
Storage pools are difficult or impossible to change without deleting data, so make the following checks first:
- Supported layouts: confirm that the NAS supports RAID 1 or its two-drive equivalent, and note whether the option is available with the filesystem you plan to use.
- Drive compatibility: check the manufacturer’s compatibility list, interface, capacity limits, sector format and any requirements for NAS-rated drives. Do not assume that a drive that fits physically is approved for your model.
- Mixed sizes: determine how the system handles unequal drives. A mirror normally uses no more than the smaller member’s capacity, but the interface may reserve or expose space differently.
- Expansion and migration: verify whether a later move to larger drives, a different RAID level or an additional bay is supported, and whether it requires a backup, a rebuild or downtime. Expansion behavior was not established for an unspecified model.
- Filesystem and snapshots: check which filesystem and snapshot features your NAS supports. Snapshot capabilities and task limits vary by model and DSM or QTS/QuTS hero version.
- Data migration path: decide how existing files will be copied into the new pool. Creating a pool can erase the drives, so an old USB disk or another destination may be needed for the initial transfer.
A practical layout decision
| Situation | Likely choice | Reasoning | Verify first |
|---|---|---|---|
| Family or small-office files must remain available after one drive fails | RAID 1 or supported two-drive mirror | Provides redundancy across the two bays | Model support, rebuild procedure and compatible replacement capacity |
| Data is temporary and can be recreated | RAID 0 may be technically possible | Uses both drives for striped capacity, with no redundancy | That every file is genuinely disposable and backed up elsewhere if it matters |
| Maximum recoverability is the priority | Mirror plus independent versioned backups | Separates availability from recovery | Backup destination, retention, encryption and restore process |
| You expect to expand later | Manufacturer-supported mirror with a documented migration path | Some systems can replace drives or add capacity in a defined sequence | Exact model and software instructions; do not infer behavior from another NAS |
Set up the two-drive mirror safely
- Identify the exact NAS and software version. Record the model, installed operating-system version, filesystem choices and drive serial numbers. Download the current storage and backup documentation for that combination.
- Confirm drive compatibility and health. Install only drives that meet the manufacturer’s interface, capacity and compatibility requirements. Run the NAS’s health or SMART checks before putting data on the array.
- Decide the recovery objective. Write down how much recent work you could lose and how long the NAS may be unavailable. Those answers determine backup frequency and whether snapshots are worthwhile.
- Create the storage pool and volume. In the NAS storage manager, choose RAID 1 or the supported two-drive mirror, select the intended filesystem, and review the projected usable capacity before confirming. Treat the confirmation as destructive unless the interface explicitly says otherwise.
- Wait for initialization or synchronization. A mirror may be usable while it synchronizes, but the array is not fully redundant until the process finishes. Avoid unnecessary heavy workloads and keep a backup of irreplaceable data during this period.
- Create shares and permissions. Separate ordinary shares from administrative areas, apply least-privilege access, and enable the NAS security updates and multifactor authentication options that your model supports.
- Record the replacement procedure. Save the model’s instructions for identifying a failed member, selecting a replacement, starting a rebuild and checking the completed array. Record the minimum replacement capacity; a drive advertised with the same nominal size may be slightly smaller in practice.
- Configure backup before loading the NAS with unique data. A redundant array without a tested backup still leaves deletion, corruption, theft and device failure unresolved.
Build the backup plan around 3-2-1
Synology and QNAP both document the 3-2-1 approach: keep three copies of the data, on two different media types, with one copy offsite. Synology’s Knowledge Center backup article was updated May 8, 2026 and applies to DSM 7 series and legacy versions. QNAP’s HBS 3 Quick Start Guide, last modified December 11, 2024, applies to HBS 3 version 17 or later on QTS or QuTS hero. These are recommendations, not measured reliability statistics.
Count the working copy on the NAS as one copy. The mirror does not automatically count as a second independent copy because both members can receive the same deletion or corruption. Add at least one separate backup destination, and make one destination geographically or logically offsite.
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Choose destinations by recovery need
| Destination | Strength | Trade-offs and checks |
|---|---|---|
| External USB drive | Fast local restores and a relatively simple first backup | It can be stolen, encrypted or damaged with the NAS if left connected; check rotation, filesystem support, encryption and where it is stored |
| Another NAS | Large capacity and scheduled remote replication or backup | Requires a second location, network bandwidth and administration; confirm destination authentication and version retention |
| Cloud destination | Offsite storage without maintaining a second physical NAS | Review upload bandwidth, encryption, retention, restore tooling, geographic availability and recurring cost before choosing a service |
| Snapshots on the NAS | Very fast rollback for supported filesystems and workloads | They use the same device and can be lost with it; model and software limits vary, so they supplement rather than replace backups |
Synology documents tools including Hyper Backup, Snapshot Replication, Active Backup for Business, USB Copy and Cloud Sync. QNAP’s HBS documentation covers backup and restore to external devices, remote devices and cloud storage, as well as synchronization workflows. Select the tool that creates independent, versioned copies rather than assuming that a synchronization task is a backup.
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Set frequency and retention from your recovery point
There is no universal schedule that fits every two-bay NAS. Set the interval according to the amount of recent work you can afford to lose:
- Daily: a reasonable starting point for many personal and household files that change occasionally.
- More often than daily: suitable for active projects, databases or documents where a day of work is unacceptable, if the NAS and destination can sustain the schedule.
- Less often: acceptable only for data that changes rarely and can be recreated or re-entered.
Retention answers a different question: how far back must you be able to go? Keep enough versions to cover the period in which accidental deletion or corruption might go unnoticed. More versions consume destination capacity, and cloud or remote retention may affect recurring charges. Set a policy in calendar terms—for example, recent versions for operational recovery plus older periodic versions for historical recovery—then confirm that the destination has enough space.
Use encryption where the destination or transport could expose personal or business data. Store recovery keys separately from the NAS, and document who can access them. A backup that is encrypted without a recoverable key is not a usable backup.
Use snapshots and replication as a fast-recovery layer
Snapshots can restore an earlier state quickly after an accidental edit or deletion, provided the NAS filesystem and model support them. Replication can maintain a second copy on another system. Snapshot task limits and device capabilities vary by model and DSM version, so read the exact documentation for your unit.
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Snapshots remain on the NAS unless replicated elsewhere. A ransomware event with administrative access, a filesystem failure, theft or physical damage can remove both live data and snapshots. Keep an independent backup job even when snapshots are enabled, and include the snapshot schedule in your capacity planning.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Test that the backup can actually be restored
A job marked successful proves that a process completed; it does not prove that the files you need are recoverable. Perform a restore test immediately after setup and repeat it on a schedule appropriate to the value of the data.
- Select several ordinary files, a large file, a recently changed file and a file from an older version.
- Restore them to a separate test folder or spare device so the originals are not overwritten.
- Open the restored files, verify their names, permissions and timestamps, and compare checksums for files where integrity matters.
- Test the recovery path after a simulated NAS outage: confirm that you can authenticate to the destination and locate the backup without the original NAS’s share paths.
- Document the elapsed time, required credentials, encryption-key location and any manual steps. Update the runbook whenever the NAS software or backup destination changes.
Plan for common failure scenarios
One drive reports an error
Check the storage manager and health logs, identify the failed member, and avoid removing a healthy drive by mistake. Confirm that a current independent backup exists, obtain a compatible replacement, and follow the model-specific rebuild procedure. Monitor the rebuild to completion and investigate any additional warnings before treating the array as healthy.
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Use a snapshot if the required version is still available; otherwise restore the appropriate version from the backup destination into a separate location. Do not restore over the only remaining copy until you have confirmed which version is correct.
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Ransomware encrypted the shares
Disconnect affected clients and stop automated jobs that could propagate encrypted files. Preserve the NAS state for investigation, then use an offline, immutable or otherwise protected version from before the event. Snapshots may help, but do not assume they survived an administrator-level attack.
The NAS is stolen or physically destroyed
Recover from the offsite destination. A USB drive stored beside the NAS does not satisfy the geographic part of 3-2-1 if the same incident can destroy both devices.
The backup job fails
Inspect the job log for authentication, capacity, connectivity, permission and encryption-key errors. Correct the cause, run the job manually, and perform a sample restore. Do not treat an old successful run as current protection.
Operational checklist
- Two compatible drives are installed, identified and health-checked.
- The selected mirror is supported by the exact NAS model and software version.
- Projected usable capacity has been reviewed with the smaller drive and system overhead in mind.
- Expansion and migration rules are documented rather than assumed.
- A scheduled, versioned backup targets an independent device or service.
- At least one backup copy is offsite or otherwise isolated from the NAS.
- Snapshots, if used, have a documented schedule and capacity limit.
- Encryption keys, administrator credentials and recovery instructions are stored separately.
- A real restore test has succeeded for current and older file versions.
- The backup and restore process has an owner and a date for the next test.
Source-specific compatibility notes
Synology’s backup guidance covers DSM 7 series and legacy versions, while QNAP’s cited HBS 3 guide covers version 17 or later on QTS or QuTS hero. Features named by either vendor are not guarantees for every two-bay model. Before applying a procedure, match the instructions to the NAS model, operating-system release, filesystem and destination type shown in your interface. Neither vendor documentation establishes a universal usable-capacity figure, migration path, drive-compatibility matrix or cloud price for an unspecified NAS.
Quick Recap
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