The UDM Pro doesn’t come with a hard drive. However, you can add pretty much any 2.5-inch or 3.5-inch HDD. I used one of my 3.5-inch 4 TB NAS drives that I had laying around. In case you’re wondering, you could also use an SSD as long as it has a SATA interface — but it would be overkill to do so because you won’t need the fast read/write speeds solid state disks offer.