Why won't my external hard drive appear in Finder?

I have an Iomega 320GB SPHDU external hard drive which I was using as my backup disk for Time Machine and also to store other files (documents, photos, etc.). I have been using for some time now and never had any problems, however this morning, I plugged the hard drive into my MacBook Pro, which is from early 2011 and on operating system OS X 10.9.4, using the USB ports and the hard drive did not appear in Finder. I checked to see if it had appeared in Disk Utility which it does, but Disk Utility is telling me that the hard drive needs repairing. When I try to repair the hard drive, I get the message '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.' As I said I have never had any problems with it before, but this has just occurred this morning and I'm unsure why it has happened.