Backup Failure

I have a 500GB external hard drive that I have partitioned for backing up both my MacBook and my wife’s MacBook.  When I perform the backup for my MacBook, the backup works just fine, but when I attempt to backup my wife’s MacBook the message I get in Time Machine is “preparing for backup”.  The message is displayed continuously and the backup never starts.  What can be the problem and how can I resolve this issue?  Help appreciated :-).  -John

You can try forcing the backup by going to the TM icon on the notification area ( top right ) and hit rum backup

You can also verify the disk on the Mac the drive is having problems via Disk Utility