Time Machine - Unable to back up

I have recently changed my Time Machine drive to a larger Seagate USB 3.0 drive and followed the Apple directions to the letter in order to do this.

Since I have done this I keep getting the message at each scheduled back up that the back up has failed "Unable to complete back. An error occurred while creating the back up folder"

If I eject & physically disconnect the drive then connect again the first back up appears to work but then the error message comes back, the same happens if I shut the Mac down so I am getting a backup daily but only due to some manual intervention.

Any ideas please ?

Any assistance would be much appreciated.