If you search (here or just Google), you should see the answer to this - I've posted it many times - its the definition of a gigabyte. Drive manufacturers use 1,000,000,000 byte definition - so you probably have 160,000,000,000 byte drive. Others use 1024 cubed, 1024 squared time 1000 and 1024 times 1000 squared. If you measure 160 billion bytes with these definitions you get c. 149, 153 and 156 GB