the full 1tb is usable it just windows reads mb as 1024kb and the drive manufacture uses 1000kb. If that makes any sense
More precisely, the hard drive manufacturer's report their drive sizes in TB (terabytes or 1,000,000,000,000 bytes) or GB (gigabytes or 1,000,000,000 bytes).
However a lot of drive utilities and OSs report the drive capacity in TiB (tibibytes or 1,099,511,627,776 bytes) or in GiB (gibibytes or 1,073,741,824 bytes).
This discrepancy came about, because in early days of computing, 1024 (2^10) was a nice round number - and it was conveniently close to 1000 (10^3), that they were both called "kilo" (as in kilobyte). As time went by, a lot of programmers adopted the convention of MB meaning 2^20 (1,048,576 bytes), GB meaning 2^30 and TB meaning 2^40 bytes. However, each step led to the discrepancy increasing - so that there is a whole 7% difference between 1 GB and 1 GiB, and 10% difference between 1 TB and 1 TiB.
While the programmers, and RAM manufacturers liked this shortcut. It never really caught on in the magnetic storage world - after all, magnetic tape could be cut to any length, and later magnetic disks worked on the same principle.
However, by 1999, the confusion was getting ridiculous, and a number of international standards groups came up with a new naming system for the binary units.
Most up-to-date Linux distros measures the drives in GiB/TiB units, and labels the units as GiB/TiB.
Mac OS X measures in GB/TB and labels the units as GB/TB.
Windows (and most disk utiltities) uses GiB/TiB but labels them GB/TB leading to massive confusion.