Why am I Experiencing an External Hard Drive Slow-Down ?

I have a Western Digital MyBook external HD which I mostly use as a backup HD and to store photos. I am using USB 2 to connect the HD to my MacBook Pro. All of a sudden, copying a 300 MB folder via drag-and-drop to the external drive takes interminably long - like an hour. I also tried to burn a CD using data rom the external drive and process started hanging about midway through (so I forced-quit). Any suggestions? Is this a hint that the drive might be dying? Thanks!