1. ensure hard drive is powered on and connected to computer
2. turn off time machine
3. go to your backup volume, backups.backupd, "your computer name", and then select and trash "In Progress" or "Latest" (it will be the last one in your backup folder listing)
4. turn Time Machine back on
5. either wait for the next backup cycle, or what I do to be sure things are working right is force an immediate backup (control-click on TimeMachine, select "backup now").