Please ensure that the hard drive you are trying to use in your DNS are on the de-vice’s hard drive compatibility list.
For more information, please refer to the product’s support page on our website:

DNS-320L / DNS-320LW
http://www.dlink.com/uk/en/support/product/dns-320l-sharecenter-2-bay-cloud-storage-enclosure

DNS-327L
http://www.dlink.com/uk/en/support/product/dns-327l-2-bay-network-attached-storage

DNS-345
http://www.dlink.com/uk/en/support/product/dns-345-sharecenter-4-bay-cloud-storage-4000