What does it mean to reformat a hard drive?
Answer: All hard drives, removable disks, CDs, DVDs, and other media need to be formatted in order for your computer to use them. The formatting process writes the basic directory structure onto the disk so that other directories and files can be added.
On a Windows computer, disks are formatted using the FAT32 or the newer NTFS file system. On a Mac, disks are typically formatted with the HFS or HFS+ file system. Once a disk is formatted, it can only be used by a computer that can read the disk's file system. Therefore, if you want to use a Macintosh-formatted disk with a Windows computer, you would need to reformat it using a Windows-compatible file system. Macintosh computers can read most Windows-formatted disks, but to use a hard drive on a Mac, you will need to format it using a Macintosh-compatible file system.
The most important thing to know about formatting or reformatting a disk is that the formatting process erases all the information on the disk. Well, technically, it just writes a new directory structure that allows all the files to be overwritten, creating a blank disk. There are some utilities that can recover files after a disk has been formatted. But for all practical purposes, formatting a disk is the same as erasing it. This is because the disk needs to be prior to formatting.
You can format most disks with Windows using the "Format" option in the File menu after selecting a disk to format. A Macintosh computer will prompt you to initialize and format an unformatted disk within the Finder or you can use the convenient "Disk Utility" program to reformat any writable disk.