Lost Space of My Hard Disk

It sounds like a difference to the way HDD manufacturers and OS manufacturers considder 1GB.

To an HDD manufacturer, 1GB = 1000000000 bytes.
To the OS manufacturers, 1GB = 1024*1024*1024 bytes = 1073741824 bytes.

So, 60GB for the HD is 60000000000 bytes = 55.8793544769287109375 GB according to the OS.