Time machine on external WD drive not recognising its backup files

Hi there.

My mid 2010 27in Imac had been working fine with time machine until I had to move the machine to work elsewhere for a day. When I got back and plugged everything in, I ran risk repair / permisssion repair using cmd R from startup then restarted.

All good except time machine sees the disc ok but wont recognise the old backup files as its own and wants to do a new 1.5Tb backup.

I cant see whats changed but Its like Time machine has disowned its old backup files.

Any suggestions very welcome indeed.

Thanks in advance

Jim