Why SHR works with drives of different sizes
Traditional RAID-5 is limited by the smallest drive in the array — three drives of 1 TB + 2 TB + 3 TB gives you only 3 TB usable. SHR (Synology Hybrid RAID) splits each drive into equal partitions and recombines them, making use of capacity that RAID-5 wastes.
Comparison with RAID
Example: 3 drives: 1 TB + 2 TB + 3 TB
| Config | Usable | Protection |
|---|---|---|
| RAID-5 | 3 TB | 1 drive |
| SHR-1 | 4 TB | 1 drive |
| SHR-2 | 2 TB | 2 drives |
SHR-1 gives you 4 TB usable instead of 3 TB with RAID-5.
Adding a drive to SHR
Step 1: Check for an empty slot
Storage Manager → Storage Pool → check for Available slots.
Step 2: Insert the new drive
DSM will automatically detect the new drive.
Step 3: Add to the storage pool
Storage Manager → Storage Pool → Action → Add Drive
Select the new drive and confirm. DSM will start the expansion process (can take hours to days depending on drive size).
During expansion the system stays fully operational, though performance will be reduced.
Extending the volume
After the Storage Pool expansion finishes:
Storage Manager → Volume → Select volume → Action → Expand Volume
Choose how much additional space to allocate from the pool.
Monitoring progress
# SSH into the Synology
cat /proc/mdstat
# Personalities : [raid1] [raid6] [raid5] [raid4] [raidF1]
# md3 : active raid5 sda3[0] sdb3[1] sdc3[2] sdd3[3]
# 8791318528 blocks super 1.2 level 5
# [4/4] [UUUU]
# [=>...................] reshape = 5.4% (159.../2930...)
# finish=847.8min speed=57623K/sec
Important notes
Expansion time depends on drive capacity and read/write speed. Adding a 4 TB drive can take 12–24 hours. Do not power off the NAS during this process.
Existing data on the new drive: When adding a drive to SHR, DSM will erase all existing data on that drive to format it. Back up any data you need before proceeding.
SHR is not a backup: SHR protects against drive failure (hardware), not against accidental deletion or ransomware. Always maintain a separate backup following the 3-2-1 rule.
# Check drive health before adding to the pool
synodiskport -disk sdd -health