The Add Drive option cannot be selected even though you have inserted a new drive into your Synology NAS.

Check the following:

Make sure the inserted drive is healthy

Go to Storage Manager > HDD/SSD to check the status of the inserted drive.

• If the drive is in Warning or Critical status, replace it with a healthy drive.
• If the drive is processing one or multiple operations (e.g., performing Secure Erase), wait for the operations to complete.
• If the drive is in Cannot be detected by Controller status (only available on SA3200D), check the LED indicators according to the LED Indicator Table in the Hardware Installation Guide.

Make sure the inserted drive meets the drive requirements

• Drive size requirements: Refer to the help articles for DSM 7.0 and DSM 6.2, respectively.
• Drive type requirements:
  • A storage pool must be comprised of drives of the same type. The following drives cannot be mixed: SATA and SAS drives, SSDs and HDDs, or 4K native and non-4K native drives.
  • Only certain Synology NAS models support using M.2 SSDs to create storage pools.

You can go to Storage Manager > HDD/SSD to check the drive size and type. If the drive size or type does not meet the requirements, replace it with a drive that does.

Make sure the status of the storage pool you intend to expand is healthy

Go to Storage Manager > Storage (for DSM 7.0) or Storage Manager > Storage Pool (for DSM 6.2) to check the storage pool status.

• If the storage pool (or volume) is processing one or multiple operations, wait for the operations to complete.
• If the storage pool (or volume) has degraded, refer to the help articles for DSM 7.0 and DSM 6.2, respectively.
• If the storage pool (or volume) has crashed, refer to this article.

Make sure the RAID type of your storage pool allows storage expansion by adding drives

Only certain RAID types support storage expansion by adding drives. For more information, refer to this article.

• The capacity of a RAID 1 storage pool cannot be expanded by adding drives. However, you can add mirror drives to a RAID 1 storage pool or add drives to change a RAID 1 storage pool to RAID 5.¹ Refer to the help articles for DSM 7.0 or DSM 6.2, respectively.

Make sure your Synology NAS model supports using drives located in expansion units

If the newly inserted drive is in an expansion unit, make sure your Synology NAS model can be paired with an expansion unit. For more information, refer to this article.

Make sure the maximum total capacity of the volumes and LUNs on your high-availability cluster has not reached or exceeded 400 TB

Once the maximum total capacity of volumes and LUNs on your high-availability cluster has reached 400 TB, you will not be able to add drives for storage expansion. For more information, refer to the Synology High Availability's technical specifications.