Here’s what the knowledge base says to try when a Synology NAS does not recognize or detect an installed drive:
1. Make sure the drive is on the Synology Products Compatibility List.
2. If there are empty drive slots, insert the drive into another slot to see whether it appears in Storage Manager.
3. If there are no empty slots, try connecting the drive to a computer to check whether it works.
4. If you can shut down the system, remove all installed drives, put the previously missing drive into a different slot, then restart and open Synology Assistant to check its status:
- Not installed: the drive still cannot be found in other slots, so the drive itself may be faulty. Try using a different drive.
- Ready: the drive is recognized in a different slot, so the issue likely lies with the previous drive slot. Contact your local reseller for repair or replacement.
Additional general checks from the manuals:
- Remove the drive and do a visual inspection.
- Confirm the drive connector is not damaged or crooked.
- Replace the drive with a new one and reboot to see whether the issue is caused by the removed drive.
- On some models, also remove the top cover and check that the power supply and connectors on the HDD backplane are properly plugged in.
- If the issue remains unresolved, contact Synology Technical Support.
For device status context:
- If drives are detected but DSM has not been installed, double-click the device to start DSM installation.
- If DSM was already installed but the device suddenly shows Not installed, the device cannot recognize the drives.
Sources used: [9], [11], [14], [15]