The MyBook comes formatted as FAT32. FAT32 has a file size limitation of 4 GB, your backup is surely larger than that. Click the start orb, in the search box type cmd, right-click the command prompt in the results and select to run as administrator. At the command prompt type
convert [drive letter for the MyBook]: /fs:ntfs without the quotes.
This will convert the drive to the NTFS format without wiping the existing data.