My Passport will not allow backups

New ‘My passport Edge for Mac’ - recognised by computer, selected in Time Machine - no problems. However, when I select ‘Back up now’, the backup appears to start but does not get further than 11 bytes before hanging and eventually the backup box disappears. The Edge is formatted (as supplied) as Mac OS Extended (journaled) and is showing as read/write. I am running Lion. Any suggestions?

Previous My Passport Studio was very happy, and I only bought the Edge as extra backup.

Reformat the drive and retry