The model number you provided (HD-H1.0TGL/R5) is for the original Terastation. Due to operating system restrictions, the maximum size drives supported by that model is 500GB, so 750GB or 1.0TB drives will not work. We have only tested the unit with the same capacity drives in the offered capacities, such as 4x160GB (0.6 TB), 4x250GB (1.0 TB), 4x400GB (1.6 TB), and 4x500GB (2.0 TB). The newer units (TS-Hx.xTGL/R5 and HS-DHx.xTGL/R5) support up to 1.0 TB drives.