Indeed, it may, to the untrained eye...
To me, it looks like your backup drive is formatted for DOS/Windows, as FAT32. That file system has a 4gb limit per file, and your image probably went over it.
To fix, use Disk Utility to erase, using the "Mac OS Extended (Journaled)" format. Then, try again. Note that it's best to back up directly to the drive, rather than to an image...