The basics
What is RAID and why does it matter?
RAID (Redundant Array of Independent Disks) uses multiple hard drives together so if one drive dies, your data survives. Think of it as a safety net for your files.
RAID is not a backup. If you accidentally delete a file it is gone instantly from all drives. If your house floods, all drives go with it. You still need a separate offsite backup.
✓ What RAID protects against
✓ A single hard drive dying
✓ Keeping your NAS running while you replace a drive
✓ Gradual bit-rot on spinning disks
✗ What RAID does NOT protect against
✗ Accidental file deletion
✗ Ransomware or viruses
✗ Fire, flood, or theft
✗ Two drives failing at once
RAID types
Which RAID level should you use?
Do not worry about the numbers. Here is what each one means in practice:
RAID 1 — Mirror (simplest, most beginner-friendly)
2 drives, half capacity (2x4TB = 4TB usable)
✓ One drive dies, the other has a perfect copy. Dead simple.
RAID 5 — Balance of space and safety (needs 3+ drives)
3x4TB = 8TB usable — you lose 1 drive worth to parity
✓ Survives 1 drive failure. Good for most home setups.
RAID 6 / RAIDZ2 — Extra safe (needs 4+ drives)
4x4TB = 8TB usable — you lose 2 drives worth to parity
✓ Survives 2 simultaneous drive failures. Best for large builds.
Synology SHR — Smart mix (great if drives are different sizes)
✓ Automatically optimises capacity across mismatched drive sizes
Check before it breaks
How to check if your drives are healthy
Every hard drive has a built-in health report called SMART. Check it regularly — drives usually warn you before they die.
Step 1Install smartmontools
Pre-installed on most NAS systems. If not:
$
sudo apt install smartmontools
Step 2Run a health check
# List your drives first
$
lsblk
# Check the three numbers that matter most
$
sudo smartctl -A /dev/sda | grep -E 'Reallocated|Pending|Uncorrectable'
Reallocated_Sector_Ct 0 ← 0 is healthy
Current_Pending_Sector 0 ← 0 is healthy
Reallocated_Sector_Ct 3 ← drive is struggling, watch it
Uncorrectable_Sector_Ct 1 ← replace this drive NOW
If Reallocated_Sector_Ct is above 0 and climbing, back up immediately and order a replacement. If Uncorrectable_Sector_Ct is above 0, replace the drive today.
Step 3Schedule automatic checks
$
sudo crontab -e
# Short test every Sunday at 1am (~2 minutes)
0 1 * * 0 smartctl -t short /dev/sda
# Long test first of every month at 2am (~3 hours)
0 2 1 * * smartctl -t long /dev/sda
# Synology: Storage Manager → HDD/SSD → Schedule
# TrueNAS: Storage → Disks → S.M.A.R.T. Tests
Emergency
My array shows DEGRADED — what do I do?
Do not panic. A degraded array is still working — it lost one drive but is surviving on parity. Act quickly but calmly.
Do NOT power off the NAS while it is in a degraded state unless absolutely necessary. Keep it running and follow the steps below.
Step 1Identify the failed drive
$
cat /proc/mdstat
md0 : active raid5 sda[0] sdc[2]
sdb[1](F) ← (F) = Failed, this is the bad drive
# Synology: Storage Manager → HDD/SSD → look for red X
# TrueNAS: Storage → Pools → click the pool → DEGRADED
# Unraid: Main tab → red drive icon
Step 2Replace the drive and rebuild
# Remove failed drive from array
$
sudo mdadm /dev/md0 --remove /dev/sdb
# Physically swap the drive, then add the new one
$
sudo mdadm /dev/md0 --add /dev/sdb
# Watch the rebuild (can take hours for large drives)
$
watch -n 10 cat /proc/mdstat
[==>..................] recovery = 14.2% finish=180min
On Synology and TrueNAS you can do all of this through the web UI — follow the repair wizard. The commands above are for bare-metal Linux only.
Keep going