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For four equal-size drives, RAID 5 is usually the capacity-focused choice, RAID 6 is the safer choice when you need two-drive failure tolerance, and RAID 10 is the mirror-and-stripe choice when you accept losing half the raw capacity. A four-bay enclosure does not require four installed drives, and the best layout depends on your NAS model, supported software, drive compatibility, workload, expansion plans, and backup strategy.
This guide explains the trade-offs, calculates usable capacity, and gives a safe setup sequence that applies across NAS platforms without pretending that every vendor uses the same menus.
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What RAID does—and what it does not do
RAID combines multiple physical drives into a storage pool that can improve availability, capacity efficiency, or performance. The array distributes data, parity, or mirrored copies according to the selected level. If a supported drive fails, the NAS can continue operating in a degraded state while you replace and rebuild the failed member.
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A four-bay NAS has four maximum drive slots; it may be populated with two, three, or four drives. The minimum drive count and available RAID modes depend on the NAS model and its operating-system version.
Compare the RAID choices for four bays
The following figures use the vendor formulas documented by Synology and assume four drives of the same size. Usable space shown by a NAS can be lower because of filesystem metadata, reserved space, and the manufacturer’s unit conventions.
| Level | Minimum drives | Drive-failure tolerance | Approximate capacity with four equal drives | Best fit |
|---|---|---|---|---|
| RAID 5 | 3 | 1 drive | About 3 drives’ worth: (N − 1) × smallest drive | Maximum capacity among these redundant classic layouts |
| RAID 6 | 4 | 2 drives | About 2 drives’ worth: (N − 2) × smallest drive | Higher protection during multiple-drive failures or long rebuilds |
| RAID 10 | 4, in an even count | Up to one drive in each mirror pair | About 2 drives’ worth | Mirrored pairs with striping; failure pattern matters |
| SHR-1 | 2 or more | 1 drive | Depends on model and drive combination | Simpler Synology-managed layout, especially with mixed drive sizes |
| SHR-2 | 4 or more | 2 drives | Depends on model and drive combination | Synology-managed two-drive tolerance |
These are vendor-described properties, not an independent reliability ranking or a performance benchmark. Actual throughput depends on the NAS processor, memory, network link, firmware, drive models, and workload.
RAID 5: capacity with one-drive protection
RAID 5 stripes data and distributed parity across at least three drives. With four equal drives, one drive’s worth is consumed by parity, leaving roughly three drives’ worth for data. The array can survive one failed drive; after that, it must be repaired before another member fails.
RAID 5 is a sensible default when usable capacity matters more than two-drive protection and the NAS supports it. Rebuilds can take many hours or longer depending on drive size and system load. During a rebuild, the array is vulnerable to another failure and may run more slowly.
RAID 6: two-drive tolerance
RAID 6 uses two independent parity calculations and requires at least four drives in the cited Synology guidance. With four equal drives, roughly two drives’ worth remains available. It can tolerate two failed drives at the same time, providing more margin during a long rebuild or when several drives are aging.
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The extra parity work costs capacity and generally lowers write performance somewhat compared with RAID 5. Synology describes RAID 6 write performance as slightly lower for this reason. Read and write results still depend on the complete NAS system rather than the RAID label alone.
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RAID 10 combines mirrored pairs with striping and needs an even number of drives, at least four in the cited guidance. Four equal drives provide roughly two drives’ worth of usable capacity. It can survive one failed drive in each mirror pair, but losing both members of the same pair defeats the array.
That failure pattern is why it is inaccurate to say RAID 10 always survives any two drive failures. RAID 10 is attractive when you prefer straightforward mirrors and accept the capacity cost, or when an application benefits from its layout. Do not assume it is faster on your NAS without testing the actual workload.
Synology Hybrid RAID
SHR is Synology’s automated storage technology. On supported models, SHR-1 provides one-drive redundancy and SHR-2 provides two-drive redundancy. SHR-2 requires at least four drives. Synology says SHR can use different drive sizes more efficiently than a classic RAID layout, but the exact capacity and supported operations depend on the model and configuration.
SHR is not a universal NAS feature. If your enclosure is not a supported Synology model, look for the manufacturer’s equivalent or use the classic RAID levels your platform documents.
Why RAID 0 is unsuitable for a sole copy
RAID 0 stripes data without parity or mirrors. It may combine capacity and improve some sequential workloads, but the cited Synology guidance lists no drive-failure tolerance. If any member fails, the volume is lost. Do not use RAID 0 for the only copy of valuable files.
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Choose a level using five questions
- How many failed drives must the pool survive? Choose RAID 5 or SHR-1 for one-drive tolerance, RAID 6 or SHR-2 for two-drive tolerance, or RAID 10 when you specifically want mirrored pairs and understand the failure pattern.
- How much usable capacity do you need? Four equal drives provide about three drives’ capacity in RAID 5 and about two in RAID 6 or RAID 10. The smallest member determines classic RAID capacity, so a smaller replacement or mismatched set can leave capacity unused.
- What does your NAS actually support? Confirm the model, operating-system version, minimum drive count, expansion rules, and conversion paths in the current manufacturer documentation. A four-bay chassis does not guarantee support for every level.
- What workload will run on it? Large media files, office documents, virtual machines, databases, surveillance recordings, and many simultaneous users stress storage differently. Treat vendor performance descriptions as guidance, not as a benchmark for your workload.
- Will you expand or change the pool later? Do not assume a level can be converted in place. Plan the final layout before storing data, and check the exact model’s documented procedure.
Check drives and compatibility before creating the pool
Identify the NAS model and installed operating-system version first. Then review the maker’s compatibility list for the exact drive family, capacity, interface, and firmware requirements. Synology warns that incompatible drives can affect performance or cause data loss.
Buy drives as a matched, supported set when possible. Confirm that every drive is detected, reports a healthy status, and has no warning or incompatible-drive message before you commit to initialization. If you are mixing capacities, verify how the selected RAID technology handles the difference; classic RAID calculations use the smallest member, while SHR may use mixed sizes more efficiently on supported systems.
Decide whether the four-bay unit will start with two, three, or four drives. RAID 5 cannot be created with only two drives, RAID 6 and SHR-2 need at least four in the cited guidance, and RAID 10 requires an even number of drives.
How to create the storage pool and volume
Menu names vary by vendor and software release. The following sequence is a safe, vendor-neutral outline; use your NAS maker’s current model-specific instructions for the actual clicks.
- Record the configuration. Write down the NAS model, operating-system version, drive serial numbers, capacities, and the intended RAID level. Save this information with your recovery documentation.
- Install the drives. Power down if the manufacturer requires it, seat each drive firmly, and follow the enclosure’s bay-order guidance. Never assume that moving members between bays is harmless.
- Confirm detection and health. Open the storage or disk-management page and check that every intended drive appears with the expected capacity and a healthy status. Resolve SMART warnings, incompatible-drive alerts, or missing disks before continuing.
- Open the storage-pool wizard. In the NAS storage-management interface, choose the command to create a new storage pool. Select only the drives intended for this pool.
- Select the RAID type. Choose RAID 5, RAID 6, RAID 10, SHR-1, or SHR-2 only if the model offers it and the minimum-drive requirement is met. Read the wizard’s estimated capacity and warning about data destruction.
- Review and initialize. Confirm the member drives, redundancy level, estimated usable space, and any erase or initialization choice. Pool creation normally destroys existing data on the selected drives, so stop if the list is wrong.
- Wait for initialization or parity construction. The NAS may be usable during this process, but performance can be reduced. Keep the unit powered, cool, and connected to reliable power; do not remove a drive or reboot unless the vendor instructs you to.
- Create a volume on the pool. A storage pool is the underlying allocation layer. Synology’s workflow requires at least one volume before files can be stored. Choose the filesystem and volume size offered by your platform, then apply the configuration.
- Create shares and permissions. Add shared folders, user accounts, access controls, and network protocols only after the volume is healthy. Start with least-privilege access and test from a client computer.
- Configure backup and monitoring. Set an independent backup target, alerts for failed drives and degraded pools, and a schedule for checking drive health. Restore a sample file to verify that the backup is usable.
Expansion and changing RAID levels
Expansion is a model-, RAID-, and software-specific operation. Synology’s current documentation lists supported expansion paths for some levels, including RAID 5, RAID 6, SHR-1, and SHR-2, but not for every layout or every model. Some classic RAID pools require adding drives of an appropriate size; others may require replacing every drive one at a time and rebuilding between replacements.
Conversion is similarly restricted. Synology documents, for example, a RAID 5-to-RAID 6 conversion that requires an additional drive, while noting that some models do not support the operation. Treat a conversion as unavailable until your exact model’s documentation confirms it.
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Before expanding or converting:
- Confirm that a current, separate backup can be restored.
- Check the required drive count, minimum capacity, and compatible drive list.
- Verify that the pool and volume have enough free space for the operation.
- Schedule the work for a maintenance window and connect the NAS to reliable power.
- Do not interrupt a rebuild or remove an additional member unless the vendor explicitly directs you.
Failure, rebuild, and troubleshooting decisions
The pool reports “degraded”
A degraded pool usually means a member drive has failed or been disconnected while redundancy remains available. Confirm which bay is affected in the storage manager, check cabling or seating if the NAS reports a connection problem, and replace only the identified drive with a compatible unit that meets the documented capacity requirement. Do not initialize or format the wrong disk.
A second drive fails during a RAID 5 rebuild
RAID 5 has only one-drive tolerance. A second failure can make the pool unavailable or unrecoverable. Stop ad-hoc repair attempts, preserve the state of the remaining drives, and use your verified backup or a qualified data-recovery service. RAID cannot reconstruct data beyond its stated tolerance.
The displayed capacity is lower than expected
Compare the NAS estimate with the formula using the smallest drive, then allow for decimal-versus-binary units, filesystem metadata, reserved system space, and snapshots. A four-drive RAID 5 pool is not expected to expose four drives’ full advertised capacities.
A drive is flagged incompatible
Do not ignore the warning merely because the drive fits physically. Check the current compatibility list, firmware requirements, and the NAS maker’s policy for unsupported models. Replace it with a supported drive before creating or expanding the pool.
The NAS becomes slow during initialization or rebuild
Parity construction and rebuilding consume disk and processor resources. Defer heavy transfers, virtual machines, or media indexing until the operation finishes, and monitor temperatures and health alerts. Do not cancel the process solely because normal file operations are slower.
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- Equating bays with drives: a four-bay enclosure may have fewer than four installed members, changing which levels are possible.
- Choosing by capacity alone: RAID 5 leaves more space than RAID 6 but has less failure tolerance.
- Assuming any two failures are safe in RAID 10: both members of one mirror pair can be lost.
- Mixing unsupported drives: physical compatibility is not the same as vendor support.
- Planning expansion without checking: conversion and expansion paths differ by model and software version.
- Skipping the volume step: a pool alone may not provide a location for shared files.
- Treating redundancy as backup: maintain and test an independent copy.
A practical decision summary
Choose RAID 5 when your four equal drives need the most usable space while surviving one drive failure. Choose RAID 6 when two-drive tolerance is worth reducing usable capacity to about two drives’ worth and accepting somewhat lower write performance. Choose RAID 10 when mirrored pairs match your workload and operating preference, and you accept the same approximate capacity cost as RAID 6 with a different failure model. On a supported Synology NAS, choose SHR-1 for automated one-drive redundancy or SHR-2 for two-drive redundancy, particularly when mixed drive sizes make classic RAID planning awkward.
Whichever layout you select, verify model support and drive compatibility first, create both the storage pool and a volume, monitor the array, and keep a separate backup that you have successfully restored.
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