Backup drive ejects after TM backup

Hi

I am not sure if this has already been covered, as there are various similar posts, so please accept my appologies if it has.

I have an external 1TB WD My Passport drive which I use for TM backup's. When I plug in via USB, DriveUnlock appears. I unlock the drive, then instigate a TM backup. All goes fine, then from what I can gather, when the TM backup has finished, the drive ejects itself. There is no error messages, unmount drive issues, it just dissappears from finder. I can only assume the backup has been successful, as no error messages appear, meaning the drive wasn't ejected while backup was in progress. I have changed the USB cable, as well as tried different ports, but the same always happens. In System Preferencies/Energy Saver, I have made sure the Put HD to sleep is un ticked. Still it happens. Has anyone else had this problem, and is it fixable? I would love to keep the drive attached and backing up for the period I am working, usually 12 hours a day, rather than the once a day at the moment.

thanks