Synology Expansion Units: Expand or Buy a Bigger NAS?

Running out of drive bays does not necessarily mean it is time to replace your Synology NAS.

Synology offers expansion units that add additional drive bays to compatible systems. For current 2025-generation DiskStation Plus models, the five-bay DX525 is the primary example. It can turn a four-bay DS925+ into a system with nine physical drive bays or give an already larger DiskStation additional room for storage growth. But adding an expansion unit is not automatically better than moving to a larger NAS. The right decision depends on capacity requirements, performance, hardware age, workload growth, redundancy, and how long you expect the infrastructure to remain in service.

What Is a Synology Expansion Unit?

A Synology expansion unit is essentially an additional drive enclosure designed to connect directly to a compatible Synology NAS.

The DX525 provides five additional bays supporting both 3.5-inch SATA HDDs and 2.5-inch SATA SSDs. Drives are hot-swappable, and storage remains managed through DSM on the host NAS. Unlike purchasing another independent NAS, the expansion unit does not become a completely separate storage server with its own DSM installation. The host NAS remains responsible for managing the environment. This makes expansion units attractive when the existing NAS still provides sufficient CPU, memory, networking, and functionality but simply lacks drive bays.

Which NAS Models Support the DX525?

Expansion units are not universal accessories.

The DX525 currently supports several 25-series DiskStation models, including:

• DS725+
• DS925+
• DS1525+
• DS1825+

Synology specifically validates expansion units against individual NAS models, so compatibility should always be checked before purchasing. Older systems may use earlier expansion units such as the DX517 rather than the DX525. For example, Synology’s compatibility database lists the DX517 as the validated expansion unit for the DS918+.

Option 1: Expand Your Existing NAS

The strongest argument for an expansion unit is simple: your existing NAS is still good enough.

Imagine a DS925+ that is already configured correctly, running reliably, and providing sufficient processing power for its workloads. The only problem is that all four internal drive bays are occupied. Replacing the entire NAS would mean buying another system even though the CPU, RAM, networking, DSM configuration, applications, and existing storage remain adequate. Adding a DX525 provides five additional bays while retaining the current platform.

Synology supports online volume expansion, allowing the additional bays to become available through DSM without replacing the host system.

Expansion Units Can Reduce Migration Work

Moving to a larger NAS may involve planning a system migration, relocating drives or data, validating applications, checking configuration compatibility, and potentially scheduling downtime. An expansion unit can make capacity growth less disruptive.

The existing NAS remains in place while additional drives are introduced.

That can be particularly useful for environments storing:

• Business files
• Backup repositories
• Surveillance recordings
• Media libraries
• Archives
• Large project files

If capacity is the only constraint, expanding the existing platform can be the simpler approach.

You Don’t Necessarily Need One Giant Storage Pool

An important planning decision is whether the new drives should extend existing storage or create separate storage. Synology says expansion units can be used to increase storage capacity or create independent volumes for additional storage or backup. A separate volume can be useful for keeping workloads logically divided.

For example:

Main NAS volume: Active company data

Expansion-unit volume: Archives or secondary local backups

Another configuration might separate surveillance recordings from general business files. This can make storage management easier than placing every available disk into one enormous pool.

Be Careful About Spanning Volumes Across Chassis

The ability to create storage across the NAS and expansion enclosure does not mean it is always the best architecture. Synology specifically recommends creating storage volumes whose drives are physically located within the same NAS or expansion unit for better performance. That recommendation is important. An expansion unit introduces another physical enclosure, power supply, connection, and set of components into the storage architecture. For critical environments, keeping storage boundaries aligned with physical hardware can simplify performance and failure planning.

When an Expansion Unit Makes the Most Sense

A DX525 or equivalent expansion unit becomes particularly attractive when the existing NAS is relatively new and still meets performance requirements.

It is generally worth considering when:

• All internal bays are occupied.
• CPU performance remains sufficient.
• Current memory capacity is adequate.
• Network performance meets requirements.
• The NAS still has years of useful life.
• More drive bays are the primary requirement.
• Reconfiguring or migrating the entire environment would be disruptive.

In this scenario, replacing a capable NAS simply to obtain more bays can be unnecessary.

When Buying a Bigger NAS Is Smarter

Storage capacity is not always the real problem.

Suppose an organization has an older four-bay NAS that is simultaneously running out of storage, CPU resources, memory, and network bandwidth.

Adding five drive bays fixes only one of those limitations.

The organization still has the same:

• Processor
• Maximum memory
• Network interfaces
• Application capabilities
• Platform age

In that situation, an expansion unit may simply postpone an inevitable migration.

Moving to a larger and newer NAS can address several infrastructure bottlenecks at once.

Performance Requirements Should Influence the Decision

Capacity and performance should be planned together.

A growing environment may begin with basic file storage but eventually support virtual machines, large backups, video production, surveillance, databases, or multiple simultaneous users. Those workloads may need faster networking, more RAM, greater CPU performance, or more capable storage architecture. If the existing NAS has become the performance bottleneck, attaching more disks does not solve the underlying problem. A larger NAS with better networking and expansion capabilities may provide a stronger long-term platform.

Consider Drive Upgrades Before Adding Hardware

There is another option between “buy an expansion unit” and “replace the NAS.”

Replace existing drives with larger ones. Synology documents several methods of increasing storage capacity, including adding drives where empty bays exist, replacing existing drives with higher-capacity models where the RAID configuration supports it, using expansion units, or migrating to a system with more bays.

For example, replacing older lower-capacity disks with substantially larger drives may provide enough additional usable storage without adding another enclosure.

The economics depend on whether the existing drives still have useful life and whether those drives can be repurposed.

Think About Your Three-Year Storage Growth

One of the biggest storage-planning mistakes is solving only today’s capacity problem. Suppose a company currently uses 40 TB and grows approximately 10 TB every year. The real requirement is not simply:

“How do we add another 10 TB?”

It should be closer to:

“What architecture supports 70–80 TB plus backup and operational headroom over the next three years?”

That calculation can change whether an expansion unit makes financial sense. If one DX525 provides sufficient growth for several years, expansion may be ideal. If the business will quickly outgrow both the NAS and its expansion capacity, migrating earlier may reduce future disruption.

Don’t Forget Backup Capacity

Additional production storage creates additional backup requirements. Adding 40 TB of usable storage without increasing backup capacity can quietly weaken the organization’s data protection strategy. Storage planning should therefore consider:

Primary storage + snapshots + backup retention + offsite copies + expected growth

The expansion enclosure can itself be configured as a separate volume for local backup use, which Synology specifically identifies as one DX525 use case. However, a local expansion unit attached to the same infrastructure should not automatically be treated as the organization’s only backup. Important data still benefits from independent and preferably offsite protection.

Expansion Unit vs Bigger NAS: A Practical Decision

A simple rule can help.

Choose an expansion unit when the NAS is still strong, but you have run out of bays. Choose a bigger NAS when you have outgrown the platform itself.

• The first is primarily a capacity problem.
• The second is an infrastructure problem.

A growing business should therefore evaluate CPU utilization, RAM, networking, storage growth, backup requirements, hardware age, and future workloads before purchasing additional bays.

Plan Synology Expansion Before Capacity Becomes Critical

The worst time to design a storage expansion strategy is when a NAS is already at 95% capacity.

Organizations should monitor storage utilization and begin planning well before available space becomes critical. A properly designed growth strategy might involve larger drives, a DX525 or another compatible expansion unit, migration to a higher-bay Synology NAS, or deployment of a larger rackmount storage platform.

The right solution depends on whether the organization needs more storage inside the existing architecture or a more capable architecture altogether. Scale capacity smartly with professional large storage solutions.

About Epis Technology

Epis Technology helps businesses plan Synology storage growth before capacity becomes an operational problem. Services include Synology consultation and deployment, large storage solutions, capacity forecasting, RAID and storage-pool planning, expansion-unit design, NAS migration, 10GbE and 25GbE networking, business backups, disaster recovery, cybersecurity, and performance optimization. Epis Technology can evaluate whether expanding an existing Synology system or migrating to a larger platform provides the better long-term balance of capacity, performance, redundancy, and cost.