GParted Live: A Boot Disk ISO You Can't Afford to Be Without
If you ever need to partition or edit the partitions on your hard drives without an existing OS on the computer, then GParted Live should be in your PC toolbox. The free GParted Live is based on a live version of Linux, (i.e. one that will boot from a disc or USB drive), and the Gnome Partition Editor, a.k.a. GPartEd, or more commonly GParted. GParted Live boots quickly, and handles virtually any partition type, including nearly all Linux, OS X, and Windows types.
Available options will vary slightly by the file system your OS uses, but in most cases GParted will detect existing partitions as well as let you create, read, expand or shrink, move, copy, check and rename them. I've been using the USB bootable version for over a year now and it has quite often booted when my Linux-based favorite, Parted Magic (which also uses GParted), wouldn't. Note, however, that version 6.0 of the latter has replaced its GUI-based boot menu with a character-based (DOS-like) boot menu and now boots more reliably. I favor Parted Magic because it doesn't require any command line intervention (GParted Live requires typing two commands) and it contains the Memtext86+ memory diagnostic tool, which I often use.
GParted is easy to use, but you need to understand partitioning to use it effectively.