time machine not backing up and hanging

After a holiday where we took the macbook with us we reconnected to the time machine and it started to do a backup.

Time machine is hanging about 40 gig into backup. Tried to leave machine but after a week nothing had improved. No back up at all. Switch the machine off and re-booted unconnected the hard drive where the previous backups were and still are. If I want anything before the 23 of August is all there.

But still no full back up done to date. The file size fo the backup is increasing. Started at 130gig now at 198.5gig.

Any suggestions welcome

back up drive is a WD my book Studio with 1T of free space.