Kingston SSD A400 120GB 240GB 480GB 960GB SATA 3.0 III 2.5” Solid State Drive. A400 SSD Solid State Drive x 1pcs/2pcs. It is calculation difference between manufacturer and your PC. Your PC says 1GB = 1.073741824 Billion bytes where Market Defines 1 GB = 1.00 Billion bytes.
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