External Hard Drive not appearing on Macbook Pro or 2009 Macbook

Hello,

I have a G Technology G-DRIVE External Hard Drive, that until now has worked perfectly.

I plugged it in today to try and back up my macbooks through Time Machine, like I have done for the last year - and on ether of the computers, the Hard Drive is not appearing in Time Machine or Finder. Having browsed online, I found that it is showing in Disk Utility and in System Information - but other than that I cannot do anthing with it. I even tried to Repair the drive, by erasing it and reformatting it - and I cant do it - I keep getting the messgae 'Error: Disk Utility can’t repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files.'

Any advice would be greatly appreciated, as I use this hard drive as a Time Machine Backup, and I'm unable to do so.