Probably because it's not formatted at all, or not formatted to the same file structure?
So try formatting the drive to NTFS (or FAT32/other) as the case may be, & try the running the restoration again. G
Hands-on: Samsung Galaxy S8 review
1995-2015: How technology has changed the world in 20 years
Samsung's beautifully designed Galaxy S8 makes for better VR experiences too
47 iPhone camera tips to help you take better photos
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.