When you re-format, it will erase ALL data on there. You may possibly add maximum extra storage space by reformatting, BUT it comes with several catches. Most of the time your OS will partition your hard drive for different uses. Most common is a recovery partition, which I would strongly recommend leaving that alone unless you have an alternate back up option in place. You can look at your hard drive partitions in Win 10 by going into disk management. If your recovery partition is for example 15Gb, then you could gain an extra 15GB of usable hard drive space by removing that partition, at the cost of having no redundancy or backup. If your hard drive is 300GB, and 15gb is reserved for recovery, then you only have ~280Gb usable. There...