1TB WD Hard Drive error

Bought  1 TB HD about 6 months ago and it had been working fine backing up to my MAC. For the past couple of weeks keep gettingfollowing two  messages:

Time Machine Error- unable to complete backup. An error occurred while copying files to the back up volume.

Time Machine has not backed up in 10 days. Please check that the back up volume is available or select a new volume in Time machine Preferences.

Would appreciate any help I can get. Has anyone experienced this problem, too? thanks in anticipation for your assistance.