Unable to format/partition hard drive

Question

So I've recently decided my previous three partitions, including a partition with windows on, are not needed so backed up the files and am trying to simply format the whole hard disk. It lets me format two of the old partitions, but not the one with windows OS on it (erase is greyed out).
In terminal I tried
diskutil reformat /dev/disk2
but get
disk2 does not appear to have a valid file system format
Usage:  diskutil reformat MountPoint|DiskIdentifier|DeviceNode
Completely erase an existing volume with the same name and format as the
original.  Ownership of the affected disk is required.
You cannot erase the boot volume, nor can you reformat any device which
does not have a valid file system (instead use diskutil eraseVolume).
The partition table is master boot record for the HDD. That's about as far as I can get. Anyone?

Answer 1

Hi there LazyGenious,
At this point you may need to boot into the Recovery HD to repartition and reformat your HDD, then reinstall OS X. 
To boot in the the Recovery HD, choose Apple menu > Restart. Once your Mac restarts (and the gray screen appears), hold down the Command (⌘) and R keys. Once booted, you should be able to launch Disk Utility. At this point you will want to repartition the HDD using the steps outlined in the User Guide below. If you are going to be reinstalling OS X, make sure to select the GUID Partition Table option.
Partition a disk - Disk Utility Help
 Partition a disk
Select the disk in the sidebar, then click Partition.
If you’re partitioning an external disk, make sure it’s connected to your computer.
Click the Partition Layout pop-up menu, then choose the number of partitions.
You can enter a size for each partition or drag the divider between the partitions to change their sizes. If a partition’s name has an asterisk beside it, it’s being shown larger than its actual size so the name can be displayed clearly.
Click each partition, then enter a name for it.
For each partition, click the Format pop-up menu, choose a format, then enter a size.
Mac OS Extended (Journaled): Uses journaling to protect the hierarchical file system integrity.
Mac OS Extended (Case-sensitive, Journaled): Uses journaling and is case sensitive to folder names. For example, folders named “Homework” and “hoMeWOrk” are two different folders.
MS-DOS (FAT): Use for Windows disks that are 32 GB or less.
ExFAT: Use for Windows disks that are over 32 GB.
Free Space: Use to extend the free space for the disk.
Click Options, then choose one of the following:
GUID Partition Table: Choose this partition scheme if you’ll be using this disk only with Mac computers.
Apple Partition Map: This partition scheme should only be used if you’re working with Mac computers using Mac OS X version 10.4 or earlier.
Master Boot Record: Also known as “fdisk,” choose this partition scheme if you’ll be using the disk as a secondary disk for the Windows operating system you installed using Boot Camp, or for an external disk you’ll be using on a Windows computer.
Click Apply.
Once this is process is complete, quit Disk Utility and reinstall OS X using the steps outlined in the article below, starting from step 2. 
OS X Mavericks: Erase and reinstall OS X
-Griff W.