Regardless of the selected RAID type, you cannot reduce an existing storage pool's number of drives or replace its drives with smaller capacity ones. Since data are distributed across the drives in most RAID types, this will affect the storage pool if you reduce or downsize the drives.¹

If you must reduce the number of drives in your storage pool, follow the steps below to delete the existing storage pool and create a new one that meets your needs:

1. Back up the data in the storage pool.
2. Do the following according to your DSM version:
   • DSM 7 and above:
     1. Go to Storage Manager > Storage.
     2. Click the upper-right icon of the storage pool you want to remove and select Remove.
     3. Read and follow the instructions on the pop-up window. Click Remove to continue.
   • DSM 6.2 and below:
     1. Go to Storage Manager > Storage Pool.
     2. Select the storage pool you want to remove and click Remove.
     3. Read and follow the instructions on the pop-up window. Click Remove to continue.
3. Enter the password of your DSM account and click Submit to confirm.
4. Click Create to create a new storage pool¹ and select the desired RAID type.
5. Follow the wizard to complete the setup.