A storage pool becomes Degraded when one or more drives becomes defective, but the storage pool remains accessible because no data loss has occurred. This only happens with RAID types that provide data redundancy, such as RAID 1, RAID 5, RAID 6, and RAID 10.
To repair it safely:
1. Open Storage Manager.
2. Go to the Overview page and click Repair.
3. Replace the failed drive with a healthy one.
4. Make sure the replacement drive is larger or equivalent to the size of the smallest drive in the storage pool.
Synology also recommends using drives from the Synology Products Compatibility List to avoid stability issues and possible data loss. If the storage pool has entered a corrupted state, repair is not possible.
Sources used: [9], [11], [12]