Swapping or expanding disks (SHR expansion)

From our experience, these are the two most common hardware events after the initial setup: either a disk has a SMART error (or has failed outright), or you simply need more storage. In both cases, thanks to SHR (Synology Hybrid RAID), it works without reformatting or data loss.

How SHR works

SHR is Synology’s own RAID layer on top of Btrfs. It combines disks of different sizes into one flexible pool and can be expanded with additional disks later — which isn’t trivial with classic RAID5/6.

• SHR1: one parity drive. One disk may fail, the data survives. Standard on DS225+ (2 bays) and small DS925+/DS1625+ setups.
• SHR2: two parity drives. Two disks may fail at the same time. Makes sense for large pools (4+ bays) and critical data.

For an expansion, we insert a new disk, DSM adds it to the volume, and redundancy is preserved.

Adding a disk for expansion

If you want more storage and your Synology has free bays:

1. You order the expansion in the customer center — you choose the HDD size (8 / 12 / 16 / 20 TB).
2. We install the disk physically in a free bay. Typically takes 1–2 business days after ordering.
3. You do the rest in DSM — no downtime, it runs in the background.

In DSM after the physical installation:

1. Storage Manager → HDD/SSD shows the new disk as “Not initialized”.
2. Storage Manager → Storage Pool → Action → Expand storage pool.
3. Select the new disk.
4. DSM warns that this takes several hours. Confirm.
5. Click Expand volume automatically.

The SHR expansion now runs in the background. With a 16 TB disk it takes 24–72 hours, depending on NAS load and volume size. During that time the NAS is slower than usual — postpone backup jobs and large file operations.

Once it’s done, the additional storage is directly available.

Replacing a failed disk

If DSM marks a disk as “failed” or reports a SMART error:

1. We notice it through our monitoring of the NAS hardware.
2. We contact you with a diagnosis and a swap plan.
3. We swap the disk during live operation (hot swap) — no downtime, RAID protects during the process.
4. DSM rebuilds automatically: the new disk is added to the RAID and the data is reconstructed.

Rebuild for a 16 TB disk: 18–48 hours. During this time your RAID is degraded — a second disk must not fail, or you get data loss. That’s the reason we recommend SHR2 for large pools.

Expanding with larger disks

If you want to replace an existing disk with a larger one (say 8 TB → 16 TB):

1. Swap the first disk: old one out, new one in. The RAID rebuilds on the new disk at its original size (8 TB usable).
2. Swap the second disk: old one out, new one in. Rebuild again.
3. Only once all disks in the RAID have the larger size can you expand the volume and use the additional storage.

With SHR this is a bit tricky, because SHR takes the smallest disk as its baseline. With two 8 TB disks + one 16 TB disk, you effectively only get 16 TB of storage (2× 8 TB as a mirror + 8 TB from the big one, the rest goes to parity). Only once all disks are larger does it work.

So if you want to convert completely (everything to a larger size), we plan that with you — usually we do it in stages over several weeks.

Migrating from SHR1 to SHR2

If you want to move to SHR2 (more safety), you need at least 4 disks. Migration:

1. Currently SHR1 with 4 disks.
2. Install an additional disk (5th bay, if available).
3. Storage Manager → Storage Pool → Change RAID type → SHR2.

The process takes several days and significantly reduces storage performance. Not to be underestimated — best done in a quiet phase.

Best practices

Mixing disk brands is fine, same sizes recommended: within a RAID, disks of the same size minimize wasted storage. Mixing brands (say Seagate IronWolf + WD Red) is actually good, because batch defects then don’t hit both at once. We use exclusively NAS-certified HDDs (tested for 24/7 operation, vibration-compensated) — which manufacturer ends up in an individual case depends on stock.

Run SMART tests routinely: in Storage Manager → HDD/SSD → Action → S.M.A.R.T. Test → Scheduled Test: set up a weekly quick test and a monthly extended test. SMART values often detect disk failures days before they actually happen — so we can swap before anything gets critical.

Before a swap or expansion: verify your latest backup. Your Hyper Backup to an external target — for example our Hyper Backup Storage — should be up to date. Even though SHR operations are normally safe: with hardware operations, a good backup is nothing to be ashamed of.

Frequently asked questions

How long does a RAID rebuild take? For a 16 TB disk: 18–48 hours. For an 8 TB disk: 8–24 hours. During the rebuild, the NAS is slower.

Can my Synology be used during the rebuild? Yes. Drive sync, browsing Photos, file access — everything works. Performance is 50–70% of normal.

What does a disk expansion cost? The HDD extra charges are on our Synology Hosting page — a monthly extra charge per disk, tiered by size. There are no one-time setup costs.

What happens if a disk fails while I’m expanding a second one? With SHR1 it gets tight — during expansion you’re already without redundancy. If a second disk fails: data loss on the pool. So: expansions preferably in quiet phases, and for critical data SHR2 anyway.

What’s the delivery time for new disks? In stock with us (standard NAS HDD sizes 8/12/16 TB): 1–2 business days until physical installation. For special sizes (20+ TB): 5–7 business days.

Can I install SSDs too? Theoretically yes, but we rarely do. NVMe cache as an accelerator makes sense, but NVMe only fits the DS925+ and DS1625+. Pure SSD pools are expensive per TB — for most private customers, 7200 RPM HDDs are more economical for the amount of data.

What if my Synology has no free bay left? Then a model change is the next step — DS225+ (2 bays) → DS925+ (4 bays) → DS1625+ (6 bays). The upgrade works like this: we migrate your data to the new Synology, you keep all configurations. If you need that: open a ticket, we’ll make a plan.

Does hot swap always work? On Plus models: yes. You can swap a disk during live operation, RAID takes over. On some older Value models, the NAS has to be off — for the models we host (Plus models), not an issue.

What’s next

• Snapshot Replication for version protection
• Planning DSM updates