There are 2.5" and 3.5" HDDs. Just in case, I backed up my laptop's HDDs to two 500gB 2.5" HDD, 1 Seagate, 1 WD. I actually cloned it using a Startech SATDOCK22RE. All my music are backed up in a 1tb 2.5" Hitachi HDD. My source has a 2tb 3.5" Hitachi HDD. I believe Hitachi's HDD division has been bought by WD.