If you’ve purchased (or have otherwise come into possession of) an external USB hard drive, and you’re having trouble getting Disk Utility to play nicely with it (i.e. reformat it to HFS+), here’s a trick for you. Open Terminal and type the following:
cd /dev/ <return>
ls - | grep disk <return>
Take note of the disks displayed. Provided you have no other external hard drives attached, your culprit disk is probably going to be disk1.
Again in the terminal, execute the following:
diskutil eraseDisk HFS+ <desired hard drive name> disk1
This invokes the Disk Utility, commands it to erase the disk disk1, rename it as <desired hard drive name> and format the drive as HFS+, or MacOS Extended Volume Hard Drive Format.
Note: Use HFS+ if you intend to use the hard drive on Mac only. If you want to use it on PC or PC/Mac, you’re going to have to choose another option. For more detailed information, check out the Apple developers website.