IMac didn’t detect my external hard drive

My external hard drive “Western Digital 500GB IBook” recently began to cause my IMac to run real slowly, forcing me to eject it. When reconnecting it, it would intermittently not mount, but when the power to the drive was disconnected and reconnected, it would (stubbornly) mount.
Recently it has refused to mount, I used "Disk Utilities" to try to correct it, but even though "Disk Utilities" detect the drive but it doesn't let me select the "Verify Disk" or "Repair Disk" buttons.
Please Help!!!
