Quick Summary (TL;DR):
Synology SHR and TerraMaster TRAID are flexible RAID-management systems designed to make mixed drive sizes and gradual expansion easier than conventional RAID. With equal-size disks, their capacity and fault-tolerance behaviour is broadly comparable to familiar parity RAID: SHR-1 / TRAID provide one-drive failure protection when enough drives are present, while SHR-2 / TRAID+ provide two-drive failure protection. Their real advantage appears when capacities differ. Standard RAID 5 and RAID 6 calculate capacity around the smallest member, so larger disks can leave unused space. SHR can divide disks into multiple underlying RAID regions and combine them into one pool; Synology’s current documentation shows this explicitly. TRAID similarly uses system-optimised capacity allocation and TerraMaster states that it can provide higher storage efficiency than conventional RAID 5 with mixed disks. The trade-off is vendor dependence: SHR is a Synology/DSM feature and TRAID is a TerraMaster/TOS feature, while standard RAID layouts are easier to understand across platforms. Expansion rules still matter: you cannot simply add any smaller disk, and changing protection level has prerequisites. For a home user who upgrades drives gradually, SHR or TRAID can be very convenient. For a fixed array of equal-size drives, standard RAID 5/6 is simpler and usually gives the same usable capacity.
SHR vs TRAID vs Standard RAID at a Glance
| Feature | Synology SHR | TerraMaster TRAID | Standard RAID 5/6 |
|---|---|---|---|
| Vendor | Synology DSM | TerraMaster TOS | Industry-standard RAID concepts |
| Mixed-size optimisation | Yes | Yes | Usually limited by smallest drive |
| One-drive protection | SHR-1 with ≥2 drives | TRAID with ≥2 drives | RAID 5 |
| Two-drive protection | SHR-2, ≥4 drives | TRAID+, ≥4 drives | RAID 6, ≥4 drives |
| Add drives later | Yes, subject to size rules | Yes, subject to size rules | RAID 5/6 commonly supported; vendor-specific process |
| Replace with larger drives | Yes | Yes | Yes on supported NAS implementations |
| Protection migration | SHR-1 → SHR-2 supported | TRAID → TRAID+ supported with conditions | RAID 5 → RAID 6 supported on many NAS platforms |
| Best advantage | Gradual mixed-size upgrades | Gradual mixed-size upgrades | Simplicity / predictable conventional layout |
Standard RAID Wastes Capacity When Drive Sizes Differ
Conventional RAID 5 and RAID 6 normally calculate usable capacity from the smallest member drive.
RAID 5 usable
≈ (number of drives − 1)
× smallest drive
RAID 6 usable
≈ (number of drives − 2)
× smallest driveConsider four drives:
6TB + 6TB + 3TB + 3TBA conventional four-drive RAID 5 effectively treats all four members as 3TB for that array:
(4 − 1) × 3TB
≈ 9TB usable decimal capacityThe extra 3TB on each 6TB disk is not automatically usable by that conventional RAID 5 group.
SHR Can Use Those Larger-Disk Regions
Synology’s current documentation explains how SHR can consist of multiple underlying RAID arrays combined into one storage pool using Linux Logical Volume Manager.
In Synology’s own example with two 4TB drives and two 6TB drives, SHR creates:
- A RAID 5 region across four 4TB sections.
- A RAID 1 region across the remaining 2TB portions of the two larger disks.
- One combined storage pool presented to DSM.
This is the core reason SHR can recover capacity that ordinary equal-member RAID would leave unused.
A Real SHR vs RAID 5 Mixed-Drive Example
Synology’s current RAID Calculator demonstrates the difference with:
6TB + 6TB + 3TB + 3TBThe current calculator reports approximately:
| Layout | Approx. available capacity shown by Synology | Approx. protection | Unused |
|---|---|---|---|
| RAID 5 | 8.2TiB | 2.7TiB | 5.5TiB |
| SHR-1 | 10.9TiB | 5.5TiB | 0 shown in this example |
The exact displayed values use Synology’s binary-capacity calculation and include system-reserved capacity, but the principle is clear: SHR can use more of the larger disks.
With Equal Drives, SHR Gives No Capacity Magic
If every drive is the same size, flexible RAID cannot create extra physical storage.
For three equal 12TB disks, Synology’s current calculator reports the same usable/protection capacity for SHR-1 and RAID 5.
Likewise, six equal 12TB disks produce the same capacity result for SHR-2 and RAID 6.
SHR’s value is flexibility—not free capacity.
What Is SHR-1?
SHR-1 is Synology’s one-drive-protection mode.
- One drive: no redundancy.
- Two or more drives: one-drive failure tolerance.
- Mixed sizes can be optimised automatically.
- Drives can be added on supported models.
- Drives can be replaced with larger models for expansion.
- SHR-1 can be migrated to SHR-2 when requirements are met.
Synology specifically recommends SHR-1 for many consumer/small NAS configurations because it simplifies storage management.
What Is SHR-2?
SHR-2 is the two-drive-protection version.
- Minimum four drives.
- Can tolerate two member-drive failures.
- Can optimise mixed-capacity drives.
- Can add/replace drives subject to DSM rules.
Conceptually, SHR-2 fills the same protection role as RAID 6 while preserving SHR’s flexible capacity allocation.
SHR Expansion Rules Still Matter
Flexible does not mean “add any drive you find”.
Current DSM guidance states that when adding a drive to SHR, it must be equal to or larger than the smallest drive already in the pool.
Synology recommends matching an existing drive size or using a drive at least as large as the largest member for better utilisation.
Mixing HDD and SSD, SATA/SAS/NVMe or incompatible sector formats in one SHR/RAID pool is also restricted.
Replacing SHR Drives with Larger Models Is Not Instant Expansion
Synology’s current 2026 guidance notes that an SHR pool made from same-size drives generally needs at least two drives replaced with larger ones before usable capacity increases.
For mixed-size SHR, replace the smallest drive first and follow DSM’s required size/order rules.
This matters when planning staged upgrades: buying one huge disk may not immediately add any usable capacity.
What Is TerraMaster TRAID?
TRAID is TerraMaster’s flexible disk-array system in TOS.
TerraMaster describes it as automatically combining disk space, providing drive-failure redundancy and automatically expanding capacity based on installed disk characteristics.
Current TOS 6 documentation lists:
- Minimum one disk.
- One-drive failure protection once the array contains at least two drives.
- System-optimised capacity.
- Higher storage efficiency than conventional RAID 5 when drive sizes allow.
- Drive addition supported.
- Online capacity expansion supported.
- Migration to TRAID+ supported when prerequisites are met.
What Is TRAID+?
TRAID+ is TerraMaster’s two-drive-redundancy flexible mode.
- Minimum four drives.
- Two-drive failure tolerance.
- System-optimised mixed-size capacity.
- TerraMaster positions it as having RAID 6-style protection with potentially better mixed-size storage efficiency.
- Adding drives and online capacity expansion are supported.
- TRAID+ itself is not listed as migratable to a higher TRAID mode.
Its role is therefore broadly analogous to SHR-2: flexible mixed-drive capacity with two-drive fault tolerance.
TRAID Also Has Drive-Size Rules
Current TerraMaster guidance says a disk added to an existing TRAID/TRAID+ array must be at least as large as the smallest disk already in the array.
A current 2026 TerraMaster support example confirms the rule: a TRAID containing 12TB members could not be expanded by adding 10TB disks because those new disks were smaller than the current minimum.
So TRAID is flexible, but it does not let you arbitrarily add smaller disks later.
TRAID to TRAID+ Migration Has Important Prerequisites
TOS supports migration from TRAID to TRAID+, but do not assume any existing TRAID can be converted immediately.
TerraMaster’s current migration guidance requires the array to satisfy drive-layout conditions. Its TOS 6 migration guide states that the TRAID must contain at least three disks of the largest capacity before conversion to TRAID+.
A January 2026 TerraMaster support case confirmed that simply adding one larger sixth disk did not satisfy the requirement for an existing mixed-size TRAID.
If two-drive protection is your eventual target, plan the disk-size progression before filling every bay.
Standard RAID Has Simpler Capacity Rules
Conventional RAID is less clever with mixed capacities, but it is extremely easy to calculate.
| RAID level | Equal/mixed drive capacity rule | Failures tolerated |
|---|---|---|
| RAID 1 | ≈ smallest drive capacity | 1 in normal 2-drive mirror |
| RAID 5 | (N − 1) × smallest drive | 1 |
| RAID 6 | (N − 2) × smallest drive | 2 |
If all disks are identical, this simplicity is a strength rather than a weakness.
When Standard RAID Is the Better Choice
- You buy all drives at once.
- All drives are the same capacity.
- You expect to replace them as a complete set.
- You want a conventional layout understood across many storage platforms.
- You do not need mixed-size capacity optimisation.
For four identical 12TB disks, RAID 5 and a one-drive-protection flexible layout offer essentially the same physical capacity. Standard RAID may therefore be the more transparent choice if flexibility has no value to you.
When SHR Is the Better Choice
- You are buying a Synology NAS.
- You expect to upgrade drives one or two at a time.
- Your drive sizes may differ.
- You want DSM to optimise mixed-capacity space automatically.
- You prefer an easier home-user expansion path.
For many home users, this is exactly the reason to choose SHR rather than manual RAID 5/6.
When TRAID Is the Better Choice
- You are using a TerraMaster NAS with current TOS.
- You expect gradual drive expansion.
- You have mixed capacities.
- You want one-drive protection without manually designing several RAID groups.
- You may later move to TRAID+ and can plan the migration prerequisites.
SHR and TRAID Create Some Vendor Dependence
SHR and TRAID are vendor-managed storage layouts.
That does not mean the underlying data is proprietary in the sense of a secret disk format—Synology, for example, documents SHR’s use of Linux md arrays and LVM—but recovery and migration are still designed around the vendor’s operating system and tools.
You should not assume an SHR set can simply be moved into a TerraMaster NAS or a TRAID set into a Synology and imported.
Standard RAID concepts are more portable conceptually, although moving an actual array between different NAS vendors can still require careful planning and may not be supported.
Flexible RAID Does Not Eliminate Rebuilds
SHR and TRAID still depend on physical disks and redundancy.
When a member fails:
- The pool becomes degraded.
- A replacement drive must be installed.
- Redundant data/parity must be reconstructed.
- The surviving disks are heavily read during repair.
Mixed-size optimisation does not remove rebuild exposure.
One-Drive Protection vs Two-Drive Protection Still Matters Most
The most important distinction is not SHR versus TRAID.
It is:
SHR-1 / TRAID / RAID 5
→ one-drive fault tolerance
SHR-2 / TRAID+ / RAID 6
→ two-drive fault toleranceFor a large six- or eight-drive NAS, the two-drive-protection class may be more important than which vendor-specific implementation you select.
Read RAID 1 vs RAID 5 vs RAID 6 for a Home NAS for the failure-tolerance trade-offs.
Equal Drives: Flexible RAID Usually Has No Capacity Advantage
If you buy four identical 16TB disks today and replace all four with identical larger disks years later, flexible RAID may provide little capacity benefit over standard RAID.
The main benefits then become simpler configuration and a potentially easier migration path—not additional terabytes.
Mixed Drives: Flexible RAID Can Save Real Money
Suppose you already own:
2 × 8TB
2 × 16TBStandard RAID 5 normally treats every drive as an 8TB member for that RAID group:
(4 − 1) × 8TB
≈ 24TB usable decimal capacityA flexible layout can potentially use additional capacity on the larger members when there are enough similarly sized larger sections to create redundant storage.
That can postpone replacing perfectly healthy smaller drives just to unlock the larger disks.
But One Huge Drive Cannot Protect Its Extra Space Alone
Flexible RAID still obeys redundancy logic.
If one disk has a unique 8TB region that no other drive can mirror or parity-protect, that region cannot magically become protected usable capacity.
This is why replacing only one drive with a much larger disk often produces little or no immediate capacity increase.
Plan Drive Upgrades in Pairs or Groups
Before buying a replacement drive, use the vendor’s capacity calculator and expansion documentation.
A sensible staged upgrade might be:
Year 1:
4 × 8TB
Year 3:
replace 2 × 8TB with 16TB
Year 4:
replace remaining 8TB drives as capacity requiresFlexible RAID can unlock capacity earlier than standard RAID, but the exact gain depends on the drive-size pattern.
Drive Cost per Usable TB Still Matters
SHR and TRAID make staged upgrades more flexible, but that does not automatically make them cheaper.
If a larger disk currently has poor £/TB value, buying it only for future expansion may still be a bad deal.
Use our current pricing method in 8TB vs 12TB vs 16TB vs 20TB NAS Drives: Cost per Usable Terabyte.
Performance: Do Not Assume Flexible RAID Is Faster
SHR and TRAID are designed primarily for capacity flexibility and easier management, not to create free performance.
TerraMaster explicitly notes that TRAID/TRAID+ parity layouts have lower write performance than non-parity RAID types such as RAID 0.
Real performance depends on:
- Drive count.
- Drive speed.
- CPU.
- Filesystem.
- Network.
- Workload.
- Underlying RAID regions.
Do not choose SHR over RAID 5—or TRAID over RAID 5—because you expect a universal MB/s improvement.
Data Scrubbing and Health Monitoring Still Matter
Flexible capacity does not reduce the need for maintenance.
- Monitor SMART/drive health.
- Run supported data scrubbing.
- Replace failing drives promptly.
- Use compatible drives.
- Protect the NAS with a UPS.
Synology and TerraMaster both provide pool-repair and scrubbing functions for supported redundant layouts.
SHR, TRAID and RAID Are Still Not Backups
SHR-2 and TRAID+ can survive two drive failures, but they cannot protect you from:
- Accidental deletion.
- Ransomware.
- Filesystem corruption.
- NAS theft.
- Fire/flood.
- Bad sync jobs.
Flexible RAID changes how efficiently disks are combined. It does not change the fundamental need for independent backup.
A Practical Decision Matrix
| Your situation | Best direction | Reason |
|---|---|---|
| Synology + mixed-size drives | SHR-1 / SHR-2 | Best DSM capacity flexibility |
| TerraMaster + mixed-size drives | TRAID / TRAID+ | TOS optimises mixed sizes |
| All drives same size | Either flexible or standard RAID | Capacity is usually similar |
| 2–4 bay beginner Synology | SHR-1 | Simple one-drive protection and expansion |
| 6–8 bay important Synology data | Consider SHR-2 | Two-drive tolerance + flexible capacity |
| 4+ bay TerraMaster needing dual protection | TRAID+ | Two-drive tolerance with mixed-size optimisation |
| Fixed identical drive set | RAID 5/6 | Simple, predictable conventional layout |
| Expect to move disks between NAS brands | Do not rely on vendor flexible RAID portability | Plan migration/backup rather than direct import |
| Weak/no independent backup | Fix backup first | No RAID mode replaces backup |
Synology SHR vs TerraMaster TRAID vs Standard RAID: The Bottom Line
SHR is one of Synology’s strongest home-user features. It can split mixed-size disks into multiple protected RAID regions and combine them into one storage pool, allowing staged upgrades to use capacity that conventional RAID may leave idle.
TRAID provides the same broad idea inside TerraMaster TOS. TRAID gives one-drive protection, while TRAID+ gives two-drive protection and both can optimise capacity based on the installed disks.
Standard RAID remains perfectly sensible when every drive is the same size. RAID 5 and RAID 6 have simple capacity rules, predictable layouts and no capacity disadvantage versus flexible RAID when all members are equal.
Flexible RAID is not unlimited flexibility. New drives must meet size rules, extra capacity needs enough peer space to be protected, and protection-level migrations have prerequisites.
The practical recommendation is simple: use SHR on Synology or TRAID on TerraMaster if you expect gradual mixed-size upgrades; use standard RAID if you prefer fixed equal-drive sets and conventional layouts. Whichever you choose, select one- or two-drive protection according to array size and availability needs, then maintain an independent backup.
Continue the Storage Protection Series
- Synology vs QNAP vs UGREEN vs TerraMaster: Which NAS Should You Buy?
- 8TB vs 12TB vs 16TB vs 20TB NAS Drives: Cost per Usable Terabyte
- RAID 1 vs RAID 5 vs RAID 6 for a Home NAS