RAID 1 vs RAID 5: Choose Bays, Usable Space and a Recovery Plan

For a home NAS with two drives, choose RAID 1: it mirrors the disks and gives you one drive's capacity. Choose RAID 5 when you need more space from at least three disks; the four-bay Synology DS425+ is our default enclosure for that plan. Either choice still needs an independent backup.

Quick picks: choose the enclosure around recovery

Compare specifications and all conditions

Keep a two-drive mirror

Pick

Synology DS225+

Order and limitation

Two drives; no third SATA bay for RAID 5

More usable space; room to grow

Pick

Synology DS425+ — default pick

Order and limitation

Three drives for RAID 5; fourth SATA bay remains free

Expansion-unit plan and ECC memory

Pick

Synology DS925+

Order and limitation

Four SATA bays; optional DX525 expands the supported total to nine

Equal-capacity drive example

Pick

WD Red Plus WD80EFPX

Order and limitation

8 TB CMR; check the exact NAS/DSM compatibility first

Full comparison table ↔

Swipe to compare. Row labels stay visible.

Your plan	Pick	Order and limitation
Keep a two-drive mirror	Synology DS225+	Two drives; no third SATA bay for RAID 5
More usable space; room to grow	Synology DS425+ — default pick	Three drives for RAID 5; fourth SATA bay remains free
Expansion-unit plan and ECC memory	Synology DS925+	Four SATA bays; optional DX525 expands the supported total to nine
Equal-capacity drive example	WD Red Plus WD80EFPX	8 TB CMR; check the exact NAS/DSM compatibility first

Your plan	Pick	Order and limitation
Keep a two-drive mirror	Synology DS225+	Two drives; no third SATA bay for RAID 5
More usable space; room to grow	Synology DS425+ — default pick	Three drives for RAID 5; fourth SATA bay remains free
Expansion-unit plan and ECC memory	Synology DS925+	Four SATA bays; optional DX525 expands the supported total to nine
Equal-capacity drive example	WD Red Plus WD80EFPX	8 TB CMR; check the exact NAS/DSM compatibility first

These are our picks by storage plan, based on official specifications. Use the NAS buying guide for the wider enclosure shortlist and NAS hard drives for disk rankings and the drive-order calculator.

RAID 1 vs RAID 5: capacity and failure comparison

RAID 1 writes identical data to its member drives; RAID 5 spreads data and parity across the array, rather than reserving one physical disk solely for parity. The table uses a two-drive mirror and equal-size disks.

The 8 TB, 16 TB and 24 TB totals are calculated capacity examples using the formulas above, before filesystem and system metadata overhead; available volume space is lower. This arithmetic predicts neither failure probability nor rebuild time.

• Two drives, RAID 18TB
• Three drives, RAID 516TB
• Four drives, RAID 524TB

Source: calculated from Synology's RAID capacity formulas and the WD80EFPX capacity

RAID 1 can also have additional mirror drives on supported systems: adding mirrors increases fault tolerance without increasing capacity. If you require two-drive failure tolerance, RAID 6 needs at least four drives; compare it before committing to RAID 5.

SHR is a separate Synology choice

Synology Hybrid RAID is Synology's automated RAID management system, with different capacity handling for mixed-size drives. A single-drive SHR pool has no fault tolerance; SHR-1 needs at least two drives for one-drive protection, while SHR-2 needs at least four for two-drive protection. Choose SHR deliberately; a generic RAID label does not describe its expansion rules.

Best for a two-drive mirror: Synology DS225+

The DS225+ specifications list two SATA bays, RAID 1 and SHR support, and a 2.5GbE port. Choose it when a mirror's usable space is enough and you expect to grow by replacing disks with larger ones. Its limit is physical: it cannot accommodate the third disk RAID 5 requires.

Best default for RAID 5: Synology DS425+

The DS425+ has four SATA bays, two M.2 slots and a 2.5GbE port; it lists RAID 1-to-RAID 5 migration. Start with three equal drives for RAID 5 and leave the fourth bay available, or start with a two-drive mirror and migrate later under DSM's requirements. The M.2 slots take NVMe SSDs, so they are not extra SATA hard-drive bays.

Variant check: these recommendations concern enclosures with drives ordered separately; check every drive model and count in a bundle. Use the disk-order calculator linked above after choosing the RAID layout.

Best for an expansion-unit plan: Synology DS925+

The DS925+ specifications list four SATA bays, 4 GB DDR4 ECC memory and two 2.5GbE ports. A separately added DX525 brings the supported bay count to nine. Choose it for those hardware requirements; budget for a separate backup destination before an expansion unit.

Variant check: Synology's shared product page also covers the DS925neo+, whose 4 GB memory is non-ECC; this pick is the DS925+ with ECC.

Example drive for the worksheet: WD Red Plus 8 TB WD80EFPX

WD lists the WD80EFPX as an 8 TB CMR SATA hard drive with 256 MB cache. Match WD80EFPX and the 256 MB cache rather than ordering on the WD Red Plus family name alone. This is a concrete capacity example, not a blanket compatibility approval.

Synology's DSM 7.3 announcement allows third-party drives for installation and pool creation on its 2025 Plus models; its specifications still guarantee full functionality only for compatible drives, and M.2 pools/cache require listed drives. Confirm the NAS, DSM version and exact drive on the current compatibility list before buying.

One failed drive: degraded is a repair window

Synology defines a degraded pool as still accessible after a drive becomes defective. With the two-drive mirror or RAID 5 arrangements here, one failed member consumes the entire drive-failure allowance. RAID 5 must reconstruct missing data and Synology warns of severely affected read and write performance while degraded. Do not plan essential work around normal performance during recovery.

1. Confirm the failed drive in Storage Manager → HDD/SSD; match its identity before removing anything, and check your independent backup.
2. Use a healthy replacement meeting DSM's rules: RAID 1 and RAID 5 repair require at least the smallest member's capacity; match drive type and sector format too.
3. Follow the enclosure manual for replacement; power down if hot-swapping is unsupported, and expect the replacement disk's existing data to be erased.
4. Select the degraded pool and Repair, then wait for a healthy state before another planned replacement.

A Crashed pool is different: Synology says you cannot repair it yourself through this procedure. Stop this workflow and use the linked support guidance and your restore plan.

Expansion: check the operation, not just the disk size

Migration: DSM permits a two-drive RAID 1 pool to become RAID 5 with an additional drive; it does not offer the reverse conversion. The pool must be healthy, and added drives must match type and be at least as large as the smallest member. Some models restrict migration or drive addition when pool members are absent from Synology's compatibility list.

Larger disks: with equal-size RAID 1 or RAID 5 members, replacing only one with a larger disk leaves the smallest-disk capacity limit unchanged. Replace Drive requires an unused drive; a full enclosure instead uses the documented deactivate/replace/repair route. Back up before expansion, and perform replacements one at a time.

Independent backup: rehearse the restore before failure

RAID is not backup: accidental deletion, corruption and ransomware are reasons to retain recoverable versions elsewhere. Synology recommends the 3-2-1 rule: three copies, two media, one copy off-site; a local-only copy cannot cover theft or fire affecting all equipment. Hyper Backup supports external devices, another Synology, rsync and cloud destinations, plus multiple versions.

For file-backup recovery after losing a pool, prepare working replacement NAS storage, then open Hyper Backup → Restore → Data → Restore from existing repositories, choose the backup destination and follow the wizard. For a restore drill with selected files, open Backup Explorer, choose a dated version, and use Copy to... into a separate test folder; check the restored files yourself. Copy to... and Download do not preserve original privileges, so rehearse permissions separately if they matter.

Keep the encryption password/key somewhere accessible independently: encrypted backups need it to restore. The .hbk repository needs Hyper Backup, Hyper Backup Explorer or Hyper Backup Vault; desktop Explorer supports Windows, macOS, Ubuntu and Fedora. Schedule integrity checks as well as rehearsing a restore.

1. 1

   Find the repository

   Record the separate destination and the restore tool.

2. 2

   Unlock and select a version

   Keep the encryption key accessible independently.

3. 3

   Restore to a test folder

   Open the recovered files and check permissions.

For power protection, the US 120 V CyberPower CP1500PFCLCD provides 1500 VA / 1000 W capacity and USB monitoring. DSM's UPS support can stop services and unmount volumes for a safe shutdown; confirm compatibility and configure it using our router and NAS UPS guide.

Buying worksheet: finish these blanks before ordering

• Space: files to retain ___; growth allowance ___; required usable capacity ___.
• Layout: RAID choice ___; enclosure ___; disks now ___; free bays later ___.
• Repair: compatible replacement model/capacity ___; where to get it ___.
• Recovery: independent destination ___; retained versions ___; key location ___; last successful restore ___; acceptable downtime ___.

If any recovery blank is unanswered, finish that plan before ordering more array capacity. Take the completed layout to the existing drive-order calculator; count backup storage separately.

How we choose

We compare official specifications and DSM documentation for bay counts, RAID support, replacement rules and restore options. These are research-based recommendations; no physical performance testing informed this guide. Capacity examples show arithmetic only. The enclosure shortlist stays within Synology so its documented repair steps have a clear scope; use the wider NAS guide for other platforms. See How we choose.

Official source notes

The main references are Synology's RAID definitions, repair requirements, migration rules, drive replacement instructions and Hyper Backup restoration guide, alongside the maker specifications linked with each pick.

Verdict

Choose the DS225+ with two drives in RAID 1 when one disk's nominal capacity meets your plan. Choose the DS425+ with at least three drives in RAID 5 for more usable space and a spare bay when starting with three. Choose the DS925+ for ECC memory or its documented expansion-unit support. For every layout, fund the independent backup and complete a restore drill.