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For four equal-size drives, RAID 5 is the usual capacity-first choice, RAID 6 is the safer choice when you need two-drive fault tolerance, and RAID 10 uses about half the raw capacity in exchange for mirrored pairs. On supported Synology systems, SHR-1 simplifies mixed-drive layouts and SHR-2 provides two-drive tolerance. The correct choice depends on the NAS model, supported modes, drive compatibility, capacity target, workload, and how many drive failures you must survive.
This guide explains the trade-offs, gives a vendor-neutral setup procedure, and shows where expansion or RAID-level changes may not be available. RAID redundancy is not a backup: keep an independent copy of important data and test that it can be restored.
What RAID changes on a four-bay NAS
RAID combines multiple physical drives into a storage pool that the NAS presents as one logical storage area. Depending on the level, some capacity is used for mirrored data or parity information so the pool can continue operating after a drive fails. RAID does not protect against accidental deletion, malware, theft, fire, a failed NAS, or a corrupted file that is replicated across every member drive.
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A four-bay enclosure has room for up to four drives; it does not mean all four bays are populated. The formulas below assume four equal-size drives. If one drive is smaller, classic RAID calculations are based on the smallest member, and the NAS may report less usable space after filesystem and system overhead.
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The three decisions that matter
- Usable capacity: how much space remains after redundancy consumes capacity.
- Failure tolerance: how many failed drives the pool can withstand without data loss.
- Performance and flexibility: how parity or mirroring affects workloads and whether the pool can later be expanded or converted.
Synology’s Knowledge Center describes these properties for its supported implementations, but another manufacturer’s firmware may use different names, restrictions, or conversion rules. Confirm the exact model and operating-system version before creating a pool.
RAID levels available on four-drive systems
| Level | Minimum drives in cited Synology guidance | Failure tolerance | Approximate capacity with four equal drives | What it means in practice |
|---|---|---|---|---|
| RAID 5 | 3 | One failed drive | About three drives’ worth: (N − 1) × the smallest drive | Best capacity efficiency among these classic redundant choices, but only one-drive tolerance. |
| RAID 6 | 4 | Two failed drives | About two drives’ worth: (N − 2) × the smallest drive | More protection during multiple failures or a rebuild, at the cost of capacity and somewhat lower write performance than RAID 5. |
| RAID 10 | Four or more, with an even number | Up to one failed drive in each mirror pair; two failures in the same pair can destroy the array | About two drives’ worth, or half the number of drives’ raw capacity | Mirrors are striped together. The exact failure pattern matters, so it cannot be described simply as “two-drive tolerance.” |
| SHR-1 | Two or more for redundancy | One failed drive | Model- and configuration-dependent | Synology’s automated layout, designed to use different drive sizes more efficiently on supported models. |
| SHR-2 | Four or more | Two failed drives | Model- and configuration-dependent | Synology’s automated two-drive-fault-tolerance option on supported models. |
These are vendor-described configuration properties, not an independent reliability ranking or a performance benchmark. Synology’s May 20, 2026 RAID guidance notes that actual performance depends on the NAS hardware, firmware, network, and workload.
Which RAID level should you choose?
Choose RAID 5 when capacity is the priority
RAID 5 leaves roughly three of four equal drives’ capacity available and tolerates one failed drive. It is a sensible classic RAID choice when the NAS supports it, you have a reliable backup, and your risk assessment accepts the possibility of a second failure before recovery completes. RAID 5 requires at least three drives in the cited Synology guidance, so a four-bay unit can start with three and add another later only if the model supports that operation.
Do not treat the extra bay as automatic expansion. The NAS may require a specific expansion procedure, a minimum number of drives, or a compatible drive size.
Choose RAID 6 when two-drive tolerance matters
RAID 6 reserves the equivalent of two drives for parity, leaving about two drives’ capacity from four equal drives. It tolerates any two failed drives, which gives more margin if a second drive fails while the first is being replaced or rebuilt. Synology describes RAID 6 write performance as slightly lower than RAID 5 because it calculates additional fault-tolerance information; the real result depends on the NAS and workload.
RAID 6 requires at least four drives in the cited guidance, so all bays must be populated in a four-bay chassis. It is often the more conservative choice for important data when the capacity cost is acceptable.
Choose RAID 10 when you prefer mirrored pairs
RAID 10 creates mirrored pairs and stripes data across those pairs. Four equal drives provide about two drives’ capacity. The array can survive one failed member in each mirror pair, but losing both members of the same pair defeats the array. Therefore, RAID 10 is not guaranteed to survive any two drive failures.
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RAID 10 can be attractive when you want a straightforward mirror-and-stripe layout and accept a 50 percent raw-capacity cost. Do not assume it will be faster than every parity layout: measure or consult the NAS maker’s specifications for your particular hardware and workload.
Choose SHR-1 or SHR-2 on a supported Synology NAS
Synology Hybrid RAID (SHR) automates the underlying arrangement and can make better use of differently sized drives than a classic level, subject to the model and the exact drive combination. SHR-1 tolerates one failed drive. SHR-2 tolerates two and requires at least four drives in Synology’s guidance.
SHR is not a universal NAS feature. A non-Synology system may not recognize an SHR pool, and even Synology models differ in supported operations. Synology’s Knowledge Center states that “RAID (classic and SHR) is not a backup solution.” Treat SHR as a convenience and capacity-management feature, not as a replacement for backups.
Avoid RAID 0 for the sole copy of valuable data
RAID 0 stripes data without redundancy. The cited Synology guidance lists no drive-failure tolerance. A single failed member can make the entire pool unavailable, so RAID 0 is inappropriate for irreplaceable files unless another complete copy exists and the loss of the pool is acceptable.
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Check compatibility before touching the drives
RAID creation normally initializes or reformats the selected drives. Read the confirmation screen carefully and move any existing data elsewhere before proceeding. Use this checklist first:
- Record the NAS model, firmware or operating-system version, and number of installed bays.
- Open the manufacturer’s current RAID and storage-pool documentation for that exact model.
- Confirm which levels are supported. A four-bay enclosure may not offer every level listed in a general article.
- Check the vendor’s drive compatibility list, including interface, capacity, sector format, and any restrictions on mixing models.
- Decide whether the drives are equal-size or mixed-size. Classic RAID generally uses the smallest drive as the capacity baseline; SHR may use mixed sizes more efficiently on supported systems.
- Plan an independent backup before creating the pool. A backup must be stored separately from the NAS and tested by restoring representative files.
- Allow time for initialization, parity synchronization, or a rebuild. The NAS may be usable during these operations, but performance and protection status can change.
Synology warns that incompatible drives can affect performance or cause data loss. That warning is model-specific vendor guidance, but the underlying rule applies broadly: use the current compatibility information for your own NAS rather than relying on a generic drive list.
How to set up RAID on a four-bay NAS
The labels and menu sequence vary by vendor and software version. The following procedure is a safe outline; use your NAS maker’s current help for the exact buttons.
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- Identify the platform. Write down the model and operating-system version. Confirm the supported RAID levels, expansion paths, and drive compatibility list.
- Install the selected drives. Power down if the manufacturer requires it, seat each drive in its bay, and start the NAS. In the storage-management utility, verify that every drive appears with the expected model, capacity, and healthy status.
- Create a storage pool. Open the storage or disk-management application, choose the command to create a pool, select the drives, and select RAID 5, RAID 6, RAID 10, SHR-1, or SHR-2 as appropriate. Read the warning about initialization and confirm that the selected drives may be erased.
- Review the estimated capacity and protection. Compare the displayed usable space with the formulas above. A lower number can be normal because of the smallest drive, metadata, reserved system space, or vendor reporting. Stop if the selected level or capacity is not what you intended.
- Start creation and wait for synchronization. Let the wizard initialize the pool. Do not remove a drive, reboot unnecessarily, or begin a drive replacement unless the vendor’s procedure explicitly allows it. Monitor the pool status for errors.
- Create a volume on the pool. A storage pool is not automatically a file system. Synology’s workflow requires at least one volume before files can be stored. Select the pool, choose the volume type and size offered by the interface, and allow formatting to complete.
- Create shared folders and accounts. Set permissions, enable the file protocols your clients need, and apply quotas or snapshots only after confirming that the volume is healthy. These options vary by operating system.
- Configure backup jobs. Copy important data to a separate NAS, external disk, cloud target, or other independent destination. Include application data and configuration exports when relevant, then perform a test restore.
- Record the layout. Save the RAID level, drive serial numbers, pool and volume names, firmware version, and backup location. This information speeds recovery if a drive or the NAS fails.
A Synology-specific note about the workflow
On supported Synology models running DSM, the Storage Manager application separates the storage pool from the volume created on it. The names of the controls can change between DSM releases, so follow the help page for the installed version rather than an old screenshot. Select SHR-1 or SHR-2 only when the model offers it and you understand how mixed drive sizes will be allocated.
Synology’s official documentation is useful for Synology hardware, but its recommendations should not be presented as universal NAS instructions. Another vendor may call a pool a storage group, expose RAID creation under a different menu, or restrict migration and expansion differently.
Capacity calculations and what the NAS will display
For four equal drives of nominal capacity D:
- RAID 5: approximately 3 × D before filesystem and system overhead.
- RAID 6: approximately 2 × D before filesystem and system overhead.
- RAID 10: approximately 2 × D before filesystem and system overhead.
Manufacturers label drives in decimal terabytes, while operating systems may display tebibytes, so the number shown in the interface can look smaller even before RAID overhead. A pool built from unequal drives is constrained by the smallest member in classic RAID. SHR’s result is configuration-dependent; use the NAS’s own estimator after selecting the actual drives.
Do not fill a pool to its headline number without leaving room for the operating system, snapshots, recycle bins, package data, and future growth. The amount reserved for those functions differs by platform and configuration, so there is no single universal percentage to apply.
Expansion and changing the RAID level
Never assume that a four-bay NAS can change RAID levels or add drives after creation. Synology’s DSM 7 help lists selected conversion paths and prerequisites; for example, converting RAID 5 to RAID 6 requires an additional drive, and some models do not support that conversion. The command may be unavailable even when the target level is valid for a newly created pool.
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Before an expansion or conversion
- Confirm the operation is listed for the exact model and current firmware.
- Make and verify a current independent backup.
- Check that every replacement drive is compatible and at least as large as required.
- Plan for reduced performance and a period of reduced redundancy while the pool rebuilds.
- Do not remove a second drive from a RAID 5 pool while it is rebuilding unless the vendor specifically instructs you to do so.
What to do when a drive fails
- Confirm the alert. Use the storage manager to identify the failed bay and verify that the warning is not a loose carrier, cable, or temporary detection problem.
- Check the protection state. RAID 5, RAID 6, RAID 10, and SHR have different limits. Do not remove a healthy drive to “test” the array.
- Obtain a compatible replacement. Follow the vendor’s minimum-capacity and compatibility rules. A replacement that is nominally the same size may be slightly smaller in actual sectors and rejected.
- Replace only the identified drive. Use the documented hot-swap procedure if supported; otherwise shut down safely.
- Start and monitor the rebuild. Select the repair or rebuild command in the storage manager and watch for errors, temperature warnings, or additional failed drives.
- Verify the pool and backups after recovery. A healthy status means redundancy has been restored; it does not validate every file. Check backup jobs and perform a restore test.
RAID 5 can remain online after one failed drive, but a second failure before recovery completes can take the pool offline. RAID 6 provides two-drive tolerance. RAID 10’s outcome depends on whether the failed drives belong to different mirror pairs.
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Troubleshooting common setup problems
The expected RAID option is missing
Check the model, firmware version, minimum-drive requirement, and whether the interface is creating a pool or a volume. RAID 6 and SHR-2 require at least four drives in the cited Synology guidance. Some systems expose only the levels supported by the selected drive count.
The displayed capacity is lower than expected
Recheck the smallest drive, decimal-versus-binary units, parity or mirror overhead, and space reserved by the operating system. For SHR, use the model-specific capacity estimate rather than a classic RAID formula.
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A drive is rejected as incompatible
Compare its exact model and firmware with the NAS maker’s current compatibility list. Check sector format and usable capacity, not just the printed terabyte rating. Do not force a drive into the pool when the vendor warns that it can cause performance or data-loss problems.
The pool is degraded after installation
Look for a drive that is not fully seated, a failed disk, or a synchronization process that has not completed. Read the event log, reseat only when the vendor permits it, and avoid removing another drive from a degraded RAID 5 or SHR-1 pool.
The volume cannot be created
Verify that the storage pool finished initialization and has available capacity. On systems that separate pools and volumes, creating the pool alone does not create a usable file system.
Backup, monitoring, and maintenance
Set alerts for failed drives, degraded pools, high temperatures, and synchronization errors. Keep spare compatible capacity available when downtime matters, but do not install a spare into the pool unless the storage manager supports that role. Schedule backups independently of RAID and periodically restore files to confirm that the backup is usable.
Before firmware updates, drive replacements, or RAID conversions, export the NAS configuration and read the release-specific instructions. Keep a written record of the pool layout and the order of mirror pairs if using RAID 10. A UPS can reduce the risk of abrupt power loss during writes, but it does not replace backups or protect against hardware failure.
A practical decision checklist
- Need about three drives’ capacity from four equal drives and can tolerate one failure? Consider RAID 5.
- Need two-drive tolerance and accept about two drives’ capacity? Consider RAID 6.
- Prefer mirrored pairs and accept about half the raw capacity? Consider RAID 10, while accounting for failure patterns.
- Use mixed-size drives on a supported Synology model? Evaluate SHR-1; choose SHR-2 when two-drive tolerance is required and the model supports it.
- Have not confirmed compatibility, expansion, or backup recovery? Do that before creating the pool.
The safest final choice is the level your specific NAS supports, your drives can reliably populate, and your backup plan can recover from. Capacity, failure tolerance, performance, and future expansion are separate decisions; no RAID level maximizes all four.
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