Synology M.2 SSD Restrictions After the Drive Policy Reversal
Synology’s 2025 drive compatibility policy caused considerable frustration among NAS users, particularly people accustomed to choosing their own hard drives and SSDs. DSM 7.3 later restored much of that flexibility for the 2025 DiskStation Plus Series, making third-party HDDs and 2.5-inch SATA SSDs considerably easier to use.
But there is an important detail that can easily get lost in the discussion.
The policy change did not remove the restrictions surrounding M.2 NVMe SSDs.
If you own a DS725+, DS925+, DS1525+, DS1825+, or another recent Synology system and want to add NVMe storage, you still need to understand the difference between HDD compatibility, SATA SSD compatibility, SSD caching, and M.2 storage pools before buying drives.
What Did Synology Actually Change After the 2025 Drive Controversy?
The original 2025 policy placed tighter restrictions on drive compatibility for new Synology systems. With DSM 7.3, Synology changed course for the 2025 DiskStation Plus Series.
On DS Plus models covered by the current policy, an HDD that is not on Synology’s compatibility list can be used for a new installation and storage pool creation, provided it is not specifically listed as incompatible.
The same flexibility applies to unlisted 2.5-inch SATA SSDs on these DS Plus systems. They can be used for new installations, storage pools, cache creation, and migration.
That is a significant improvement for users who want more choice when purchasing storage.
However, M.2 NVMe SSDs are treated differently.
Third-Party M.2 NVMe Drives Remain Restricted
Synology’s current drive compatibility policy makes an explicit distinction between 2.5-inch SATA SSDs and M.2 NVMe SSDs.
For DS Plus and desktop FS models released in 2025 and later, an M.2 NVMe SSD that appears on the compatibility list is supported according to the capabilities of the NAS.
An M.2 NVMe SSD that is not listed, however, is not supported for:
- New installation and storage pool creation
- SSD cache creation
There is a limited migration exception for qualifying drives moved directly from an existing Synology system, but that should not be confused with general support for installing a new third-party NVMe SSD.
So if you buy an inexpensive consumer NVMe drive expecting DSM 7.3 or DSM 7.4 to treat it like an ordinary SATA SSD, you may discover that the option you wanted is unavailable.
Why Does Synology Treat M.2 SSDs Differently?
Synology says the distinction comes down largely to workload.
M.2 NVMe drives in Synology NAS systems are commonly used for caching, where they may experience sustained random I/O and continuous read/write activity.
That can be substantially different from typical consumer PC usage.
Synology argues that many general-purpose M.2 SSDs are designed around lighter or burst-oriented workloads and may have inadequate endurance, thermal behavior, or sustained performance for NAS caching.
For that reason, Synology says M.2 NVMe SSDs used for storage pools and cache must meet its performance, endurance, and thermal requirements.
Whether users agree with the restriction or would prefer to accept that risk themselves is a separate question. From a practical purchasing perspective, however, the important fact is that the restriction remains.
M.2 Storage Pools Have Another Limitation: Your NAS Must Support Them
Even a compatible M.2 SSD does not automatically mean you can create an NVMe storage pool.
Only certain Synology NAS models support M.2 SSD storage pools.
Synology’s published list includes several recent Plus systems, including:
- DS725+
- DS925+
- DS1525+
- DS1825+
- DS425+
- DS723+
- DS923+
- DS423+
- DS1522+
- DS1621+
- DS1821+
- Selected other compatible models
Having an M.2 slot therefore does not necessarily mean that DSM allows that slot to become a normal storage volume.
Some Synology models support M.2 primarily for SSD caching.
Always check the exact model rather than assuming all NVMe-equipped DiskStations have identical functionality.
Synology Still Specifies Its Own M.2 SSDs for Storage Pools
Synology’s current Knowledge Center documentation for M.2 storage pools specifically states that supported systems use Synology M.2 NVMe SSDs that are compatible with the NAS model.
This remains an important difference from the more relaxed policy for third-party HDDs and 2.5-inch SATA SSDs.
There are other limitations too.
DSM itself cannot be installed entirely onto the M.2 SSD storage pool. Synology requires at least one non-M.2 drive in the NAS for DSM installation.
M.2 drives are also not hot-swappable. If you need to physically install or remove one, the NAS needs to be shut down.
That matters when planning a system where the NVMe volume contains important applications or frequently accessed data.
SSD Cache Is Not the Same Thing as an SSD Storage Pool
This distinction causes considerable confusion.
An SSD cache accelerates access to data stored on another volume.
An SSD storage pool uses the SSDs themselves as primary storage.
Synology DSM supports both read-only and read-write SSD caching on compatible systems.
A read-only cache primarily helps random read performance. Because it stores copies of existing data, losing the cache does not mean the original data disappears.
A read-write cache can accelerate both reads and writes, but it has greater data-protection implications and requires an appropriate redundant cache configuration.
An M.2 storage pool is different. You can create a volume on it and actually store applications or data there.
Before buying M.2 SSDs, decide which of these jobs you expect them to perform.
Don’t Assume Your NAS Actually Needs SSD Cache
Another mistake is installing M.2 drives simply because the slots are available. A Synology performance review can reveal whether caching improves workloads.
SSD caching can be useful for workloads involving frequent random I/O, but it does not automatically make every Synology workload noticeably faster.
A NAS primarily used for:
- Large sequential backups
- Movie storage
- Media archives
- Cold storage
- Large sequential file transfers
may see less benefit than a system dealing with:
- Databases
- Frequently accessed small files
- Virtualization
- Busy multi-user file services
- Repeated random reads
- Application workloads
Before spending money on Synology-compatible M.2 SSDs, determine whether the workload is actually cache-sensitive.
Sometimes additional RAM, faster networking, more capable HDDs, or a dedicated SSD storage pool can provide greater practical value.
Remember That SSD Cache Uses System Memory Too
The SSDs are not the only resource required for caching.
DSM 7.4 specifies approximately 400 KiB of system memory for every 1 GiB of SSD cache, with expandable memory included in that calculation.
That means an extremely large SSD cache can create additional RAM requirements.
This is particularly relevant on smaller DiskStation configurations that still have their original factory memory.
Buying two large NVMe SSDs without considering system RAM can therefore result in an unnecessarily oversized cache or another upgrade requirement.
What About 2.5-Inch SATA SSDs?
This is where DSM 7.3’s policy change becomes much more noticeable.
On covered DS Plus systems, third-party 2.5-inch SATA SSDs that are not on the compatibility list can generally be used for:
- New installations
- Storage pool creation
- SSD cache
- Migration from an existing Synology
That provides another option if you want solid-state storage without accepting the tighter M.2 compatibility rules.
The tradeoff is obvious: a 2.5-inch SSD consumes one of the NAS’s regular drive bays.
In a two-bay or four-bay NAS, sacrificing a drive bay to SSD storage may not be desirable.
M.2 slots are attractive precisely because they allow SSD functionality without occupying the main SATA bays, which makes the continuing NVMe restrictions more noticeable.
Can You Work Around the M.2 Restrictions?
Unofficial community modifications have historically existed for enabling unsupported NVMe functionality or changing how DSM handles drive compatibility.
For a lab or experimental system, advanced users may choose to investigate those approaches.
But they should not be confused with officially supported Synology configurations.
A DSM update can change behavior, scripts may need to be reapplied, monitoring may not behave as expected, and Synology support may not assist with problems involving unsupported configurations.
For a business NAS or a system holding important data, building the storage architecture around an unofficial workaround creates an additional maintenance dependency.
The more reliable alternatives are usually:
- Use compatible Synology M.2 SSDs
- Use supported 2.5-inch SATA SSDs
- Keep HDD storage and use M.2 only where officially supported
- Select a Synology model whose storage capabilities match the workload
- Move demanding compute or high-speed storage workloads to another server while retaining Synology for centralized storage and backup
That last option can be particularly useful in virtualization environments where Proxmox, Hyper-V, or another compute platform provides local NVMe performance while Synology remains responsible for shared storage and data protection.
Check Compatibility Before Buying SSDs, Not After
The most important practical advice is simple: check the exact NAS model, DSM version, drive model, and intended SSD function before purchasing anything.
Do not ask only:
“Will this NVMe SSD physically fit my Synology?”
Ask:
“Will this exact SSD be supported for the specific function I want on this exact NAS?”
A drive could physically fit an M.2 slot while still not being eligible for cache creation or a storage pool.
Likewise, two Synology NAS models with similar-looking M.2 slots may not support identical storage configurations.
The 2025 Drive Policy Reversal Was Significant, but It Wasn’t Universal
Synology users have considerably more drive flexibility than they did when the controversial 2025 policies first appeared.
For recent DS Plus systems, DSM 7.3 restored the ability to create storage pools using unlisted third-party HDDs and 2.5-inch SATA SSDs, as long as those drives are not specifically listed as incompatible.
That was an important change.
But M.2 NVMe remains the major exception.
Synology continues to apply stricter compatibility requirements to M.2 caching and NVMe storage pools, and only specific NAS models support M.2 storage pools in the first place.
That does not automatically make the current Synology platform a poor value. It simply means buyers need to understand exactly where the drive-policy flexibility ends.
If you primarily need high-capacity HDD storage, the policy situation is now considerably more flexible. If your planned build depends heavily on inexpensive third-party NVMe drives for cache or primary storage, however, the remaining restrictions deserve attention before you choose the NAS or purchase the SSDs.
About Epis Technology
Epis Technology helps businesses design Synology storage around real capacity, performance, reliability, and application requirements. Services include Synology model selection, DSM deployment, HDD and SSD planning, M.2 cache configuration, SSD storage pools, memory upgrades, NAS migrations, Btrfs storage, Synology Drive, virtualization storage, enterprise backup, Microsoft 365 and Google Workspace protection, ransomware resilience, networking, and disaster recovery. Epis Technology can help determine whether SSD caching or NVMe storage will actually improve a workload and select a supported configuration without unnecessary hardware purchases or compatibility problems.