I've had drives that refuse for format in Disk Utility before, easiest way to force solve this is to format it via terminal, use:
Open Terminal, and type:
diskutil list
You'll get a list of attached drives with their partitions like this:
shousiqiangdaoPro:~ james$ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 999.7 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *500.1 GB disk1
1: EFI EFI 209.7 MB disk1s1
2: Microsoft Basic Data G-Drive 500 499.9 GB disk1s2
shousiqiangdaoPro:~ james$
Locate the path to your drive (in bold) (and be very careful to get the right disk, I would disconnect any drives you don't want to accidentally erase)
In my case my G-Drive500 is /dev/disk1
Then simply type in this format:
diskutil eraseDisk JHFS+ [NAME YOU WANT DISK TO BE CALLED WITH NO SPACES] [DISK PATH]
So for me that becomes:
diskutil eraseDisk JHFS+ G-Drive500 /dev/disk1