Can't erase/unmount external Seagate hard drive

I have a Seagate external Backup Plus drive (1.5 TB) that worked fine with Time machine for at least 6 months using my Macbook Pro (8 GB, 1600 MHz DDR3 (Core i5), OS X (10.9.5).

It recently stopped all Time machine backups and I can't access the drive although it shows up in the disk utility left hand column. I need to return it to Seagate as it is covered by the warranty and I want to erase the disk before it goes back but I can't get any disk utility program to erase or unmount the disk. I've gone into recovery mode, tried a few other apps but nothing seems to work. Any help would be greatly appreciated! Thx.